Implantable medical device crosstalk evaluation and mitigation

ABSTRACT

Electrical crosstalk between two implantable medical devices or two different therapy modules of a common implantable medical device may be evaluated, and, in some examples, mitigated. In some examples, one of the implantable medical devices or therapy modules delivers electrical stimulation to a nonmyocardial tissue site or a nonvascular cardiac tissue site, and the other implantable medical device or therapy module delivers cardiac rhythm management therapy to a heart of the patient.

This application claims the benefit of U.S. Provisional Application No.61/110,239, entitled, “IMPLANTABLE MEDICAL DEVICE CROSSTALK EVALUATIONAND MITIGATION,” and filed on Oct. 31, 2008, the entire content of whichis incorporated herein by reference.

TECHNICAL FIELD

The disclosure relates to therapy systems, and, more particularly,therapy systems including at least two therapy delivery modules.

BACKGROUND

A wide variety of implantable medical devices that deliver a therapy ormonitor a physiologic condition of a patient have been clinicallyimplanted or proposed for clinical implantation in patients. Someimplantable medical devices may employ one or more elongated electricalleads and/or sensors. Such implantable medical devices may delivertherapy or monitor the heart, muscle, nerve, brain, stomach or otherorgans. In some cases, implantable medical devices deliver electricalstimulation therapy and/or monitor physiological signals via one or moreelectrodes or sensor elements, at least some of which may be included aspart of one or more elongated implantable medical leads. Implantablemedical leads may be configured to allow electrodes or sensors to bepositioned at desired locations for delivery of stimulation or sensingelectrical depolarizations. For example, electrodes or sensors may belocated at a distal portion of the lead. A proximal portion of the leadmay be coupled to an implantable medical device housing, which maycontain electronic circuitry such as stimulation generation and/orsensing circuitry. In some cases, electrodes or sensors may bepositioned on an IMD housing as an alternative or in addition toelectrodes or sensors deployed on one or more leads.

For example, implantable cardiac devices, such as cardiac pacemakers orimplantable cardioverter defibrillators, provide therapeutic electricalstimulation to the heart by delivering electrical therapy signals suchas pulses or shocks for pacing, cardioversion or defibrillation pulsesvia electrodes of one or more implantable leads. In some cases, animplantable cardiac device may sense intrinsic depolarizations of theheart, and control the delivery of therapeutic stimulation to the heartbased on the sensing. When an abnormal rhythm of the heart is detected,such as bradycardia, tachycardia or fibrillation, an appropriateelectrical therapy (e.g., in the form of pulses) may be delivered torestore the normal rhythm. For example, in some cases, an implantablemedical device may deliver pacing, cardioversion or defibrillationsignals to the heart of the patient upon detecting ventriculartachycardia, and deliver cardioversion or defibrillation therapy to apatient's heart upon detecting ventricular fibrillation. Some medicaldevice systems that include a neurostimulator in addition to implantablecardiac device have also been proposed.

SUMMARY

In general, the disclosure is directed toward therapy systems thatdeliver electrical stimulation therapy to a tissue site within a patientand cardiac rhythm management therapy to a heart of a patient. Thetissue site for the electrical stimulation therapy may be, for example,a nonmyocardial tissue site or nonvascular cardiac tissue site (e.g., acardiac fat pad). In some examples, the therapy system may include afirst implantable medical device (IMD) that delivers electricalstimulation to a tissue site within a patient, such as proximate a nerve(e.g., a vagus nerve or a spinal cord) or another nonmyocardial tissuesite, and a second implantable medical device (IMD) that deliverscardiac rhythm management therapy, such as at least one of pacing,cardioversion or defibrillation therapy to a heart of the patient. Thefirst IMD may be referred to as an implantable neurostimulator (INS) oran electrical stimulator, and the second IMD may be referred to as animplantable cardiac device (ICD). The INS may deliver electricalstimulation to nonmyocardial tissue sites other than sites adjacentnerves, and the ICD may deliver any combination of pacing,cardioversion, and defibrillation pulses. In other examples, the therapysystem may include an implantable medical device that includes a firsttherapy module that delivers stimulation therapy to a nonmyocardialtissue site within a patient and a second therapy module that deliversat least one of pacing, cardioversion or defibrillation therapy to theheart of the patient, where the first and second therapy modules aredisposed in a common housing.

Techniques for minimizing interference between the INS and the ICD orbetween the different therapy modules of a common medical device aredescribed herein. In some examples, the therapy parameter values thatdefine the electrical stimulation delivered by the INS may be modifiedin order to reduce the possibility that the ICD senses the electricalstimulation signals delivered by the INS and mischaracterizes the sensedsignals as cardiac signals. In other examples, the INS may switchtherapy programs that define the electrical stimulation signalsgenerated and delivered by the INS upon the detection of an arrhythmiaby the ICD, the INS or another device. In addition to or instead ofmodifying the operation of the INS, some examples described hereinmodify one or more sensing parameter values of an ICD order to reducethe possibility that the ICD senses the electrical stimulation signalsdelivered by the INS and mischaracterizes the sensed signals as cardiacsignals.

In addition, the disclosure describes techniques for evaluating theamount of interference (or “crosstalk”) between an INS and ICD implantedwithin a patient. The measured interference may be used to modifyoperation of the INS or ICD, and, in some cases, may be recorded forlater analysis by a clinician.

In one aspect, the disclosure is directed to a method comprisingdetermining a characteristic of a first electrical signal sensed by afirst IMD while a second IMD is delivering a second electrical signal toa tissue site within a patient, wherein the first and second IMDs areimplanted within the patient, comparing the characteristic of the firstelectrical signal to a threshold value, and modifying at least one of astimulation parameter with which the second IMD generates electricalstimulation that is delivered to the tissue site or a sensing parameterwith which the first IMD senses the first electrical signal based on thecomparison.

In another aspect, the disclosure is directed to a system comprising afirst IMD that senses a first electrical signal within a patient, asecond IMD that delivers a second electrical signal to a tissue sitewithin patient, wherein the first IMD senses the first electrical signalwhile the second IMD delivers the second electrical signal to thepatient, and a processor that determines a characteristic of the firstelectrical signal, compares the characteristic to a threshold value, andmodifies at least one of a stimulation parameter with which the secondIMD generates electrical stimulation therapy or a sensing parameter withwhich the first IMD senses the first electrical signal based on thecomparison.

In another aspect, the disclosure is directed to a system comprisingmeans for determining a characteristic of a first electrical signalsensed by a first IMD while a second IMD is delivering a secondelectrical signal to a tissue site within a patient, wherein the firstand second IMDs are implanted within the patient, means for comparingthe characteristic of the first electrical signal to a threshold value,and means for modifying at least one of a stimulation parameter withwhich the second IMD generates electrical stimulation that is deliveredto the tissue site or a sensing parameter with which the first IMDsenses the first electrical signal based on the comparison.

In another aspect, the disclosure is directed to a computer-readablemedium comprising instructions that cause a programmable processor todetermine a characteristic of a first electrical signal sensed by afirst IMD while a second IMD is delivering a second electrical signal toa tissue site within a patient, wherein the first and second IMDs areimplanted within the patient, compare the characteristic of the firstelectrical signal to a threshold value, and modify at least one of astimulation parameter with which the second IMD generates electricalstimulation that is delivered to the tissue site or a sensing parameterwith which the first IMD senses the first electrical signal based on thecomparison.

In one aspect, the disclosure is directed to a method comprisingevaluating crosstalk between a first IMD implanted within a patient anda second IMD implanted within the patient with an external device. Theexternal device determines a characteristic of a first electrical signalsensed by the first IMD while a second IMD is delivering a secondelectrical signal to a tissue site within a patient, compares thecharacteristic of the first electrical signal to a threshold value, andmodifies at least one of a stimulation parameter with which the secondIMD generates electrical stimulation that is delivered to the tissuesite or a sensing parameter with which the first IMD senses the firstelectrical signal based on the comparison.

In one aspect, the disclosure is directed to a method comprisingdetermining a characteristic of a first electrical signal sensed by afirst IMD while a second IMD delivers a second electrical signal to atissue site within a patient, wherein the first and second IMDs areimplanted within the patient, determining an electrical noise statusbased on the characteristic of the first electrical signal, andgenerating a notification based the noise status

In another aspect, the disclosure is directed to a system comprising afirst IMD that senses a first electrical signal within a patient, asecond IMD that delivers a second electrical signal to a patient,wherein the first IMD senses the first electrical signal while thesecond IMD delivers the second electrical signal to the patient, and aprocessor that determines a characteristic of the first electricalsignal, determines an electrical noise status based on thecharacteristic of the first electrical signal, and generates anotification based the noise status.

In another aspect, the disclosure is directed to a system comprisingmeans for determining a characteristic of a first electrical signalsensed by a first IMD while a second IMD delivers a second electricalsignal to a tissue site within a patient, wherein the first and secondIMDs are implanted within the patient, means for determining anelectrical noise status based on the characteristic of the firstelectrical signal, and means for generating a notification based thenoise status.

In another aspect, the disclosure is directed to a computer-readablemedium comprising instructions that cause a programmable processor todetermine a characteristic of a first electrical signal sensed by afirst IMD while a second IMD is delivering a second electrical signal toa tissue site within a patient, wherein the first and second IMDs areimplanted within the patient, determine an electrical noise status basedon the characteristic of the first electrical signal, and generate anotification based the noise status.

In one aspect, the disclosure is directed to a method comprisingevaluating crosstalk between a first IMD implanted within a patient anda second IMD implanted within the patient with an external device. Theexternal device determines a characteristic of a first electrical signalsensed by the first IMD while a second IMD delivers a second electricalsignal to a tissue site within a patient, determines an electrical noisestatus based on the characteristic of the first electrical signal, andgenerates a notification based the noise status.

In one aspect, the disclosure is directed to a method comprisingdetermining a first electrical parameter value indicative of animpedance of an electrical path coupled to a first IMD, activatingdelivery of electrical stimulation to a patient by a second IMD afterdetermining the first electrical parameter value, determining a secondelectrical parameter value indicative of the impedance of the electricalpath while the second IMD delivers electrical stimulation, determiningwhether the first and second electrical parameter values are within athreshold range of each other, and generating an indication if the firstand second electrical parameter values are not within the thresholdrange of each other.

In another aspect, the disclosure is directed to a system comprising afirst IMD implanted within a patient, a second IMD implanted within thepatient, and a processor. The processor determines a first electricalparameter value indicative of an impedance of an electrical path coupledto the first IMD, determines a second electrical parameter valueindicative of the impedance of the electrical path while the second IMDdelivers electrical stimulation to the patient, determines whether thefirst and second electrical parameter values are within a thresholdrange of each other, and generates an indication if the first and secondelectrical parameter values are not within the threshold range of eachother.

In another aspect, the disclosure is directed to a system comprisingmeans for determining a first electrical parameter value indicative ofan impedance of an electrical path coupled to a first IMD, means foractivating delivery of electrical stimulation to a patient by a secondIMD after determining the first electrical parameter value, means fordetermining a second electrical parameter value indicative of theimpedance of the electrical path while the second IMD deliverselectrical stimulation, means for determining whether the first andsecond electrical parameter values are within a threshold range of eachother, and means for generating an indication if the first and secondelectrical parameter values are not within the threshold range of eachother.

In another aspect, the disclosure is directed to a computer-readablemedium comprising instructions that cause a programmable processor todetermine a first electrical parameter value indicative of an impedanceof an electrical path coupled to a first IMD, activate delivery ofelectrical stimulation to a patient by a second IMD after determiningthe first electrical parameter value, determine a second electricalparameter value indicative of the impedance of the electrical path whilethe second IMD delivers electrical stimulation, determine whether thefirst and second electrical parameter values are within a thresholdrange of each other, and generate an indication if the first and secondelectrical parameter values are not within the threshold range of eachother.

In one aspect, the disclosure is directed to a method comprisingdetermining a first characteristic of a first electrical cardiac signalof a patient sensed by a first IMD, determining a second characteristicof a second electrical cardiac signal of the patient sensed by the firstIMD while a second IMD delivers electrical stimulation to a tissue sitewithin the patient, wherein the first and second IMDs are implantedwithin the patient, determining an electrical noise status based on thefirst and second characteristics, and generating a notification basedthe electrical noise status.

In another aspect, the disclosure is directed to a system comprising afirst IMD that senses first and second electrical cardiac signals of aheart of a patient, a second IMD that delivers a second electricalsignal to a patient, wherein the first IMD senses the second electricalcardiac signal while the second IMD delivers electrical stimulation to atissue site within patient, and a processor that determines a firstcharacteristic of the first electrical signal and a secondcharacteristic of a second electrical cardiac signal, determines anelectrical noise status based on the first and second characteristics,and generates a notification based the electrical noise status.

In another aspect, the disclosure is directed to a system comprisingmeans for determining a first characteristic of a first electricalcardiac signal of a heart of a patient sensed by a first IMD, means fordetermining a second characteristic of a second electrical cardiacsignal of the heart sensed by the first IMD while a second IMD deliverselectrical stimulation to a tissue site within the patient, wherein thefirst and second IMDs are implanted within the patient, means fordetermining an electrical noise status based on the first and secondcharacteristics, and means for generating a notification based theelectrical noise status.

In another aspect, the disclosure is directed to a computer-readablemedium comprising instructions that cause a programmable processor todetermine a first characteristic of a first electrical cardiac signal ofa heart of a patient sensed by a first IMD, determine a secondcharacteristic of a second electrical cardiac signal of the heart sensedby the first IMD while a second IMD delivers electrical stimulation to atissue site within the patient, wherein the first and second IMDs areimplanted within the patient, determine an electrical noise status basedon the first and second characteristics, and generate a notificationbased the electrical noise status.

In another aspect, the disclosure is directed to a method comprisingevaluating crosstalk between a first IMD implanted within a patient anda second IMD implanted within the patient with an external device,wherein the external device, determines a first characteristic of afirst electrical cardiac signal of a heart of a patient sensed by thefirst IMD, determines a second characteristic of a second electricalcardiac signal of the heart sensed by the first IMD while a second IMDdelivers electrical stimulation to a tissue site within the patient,determines an electrical noise status based on the first and secondcharacteristics, and generates a notification based the electrical noisestatus.

In another aspect, the disclosure is directed to a computer-readablemedium comprising instructions. The instructions cause a programmableprocessor to perform any part of the techniques described herein.

The details of one or more examples of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the disclosure will be apparent from thedescription and drawings, and from the example statements providedbelow.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example therapy systemincluding an implantable cardiac device (ICD) and an implantableneurostimulator (INS).

FIG. 2 is a conceptual diagram illustrating another example therapysystem that includes the ICD and the INS.

FIG. 3 is a conceptual diagram illustrating the ICD of FIGS. 1 and 2 andthe respective leads in greater detail.

FIG. 4 is a conceptual diagram illustrating another example of the ICDof FIGS. 1 and 2 and the respective leads in greater detail.

FIG. 5 is a conceptual diagram illustrating another example therapysystem that includes an ICD and an INS.

FIG. 6 is a functional block diagram of an example ICD that generatesand delivers electrical stimulation to a heart of a patient.

FIG. 7 is a functional block diagram of an example INS that generatesand delivers electrical stimulation signals to a tissue site within thepatient.

FIG. 8 is a functional block diagram of an example medical deviceprogrammer.

FIG. 9 is a flow diagram illustrating an example technique for modifyingelectrical stimulation therapy delivered by an INS.

FIG. 10 is a flow diagram illustrating another example technique formodifying electrical stimulation therapy delivered by an INS.

FIGS. 11A-11D are flow diagrams illustrating another example techniquefor modifying electrical stimulation therapy delivered by an INS.

FIGS. 12A and 12B are flow diagrams illustrating example techniques fordelivering electrical stimulation therapy to a patient.

FIGS. 13A-13I are conceptual illustrations of example waveforms forelectrical stimulation therapy.

FIGS. 14A and 14B are conceptual illustrations of example electrodecombinations that may be used to deliver a biphasic stimulation signal.

FIGS. 15A-15F are conceptual illustrations of example electrodecombinations that may be used to help focus a stimulation fieldgenerated by the delivery of electrical stimulation by an INS.

FIG. 16 is a flow diagram illustrating an example technique that an ICDmay implement in order to detect an arrhythmia while an INS isdelivering electrical stimulation.

FIGS. 17A and 17B are conceptual illustrations of sensedelectrocardiogram (ECG) signals prior to and after a neurostimulationsignal artifact is at least partially removed from the sensed ECGsignal.

FIGS. 18A and 18B are conceptual illustrations of sensed ECG during aventricular tachycardia prior to and after a neurostimulation signalartifact is at least partially removed from the sensed ECG signal.

FIGS. 19A and 19B are flow diagrams illustrating example techniques thatan ICD may implement in order to detect an arrhythmia while an INS isdelivering electrical stimulation.

FIG. 20 is a flow diagram illustrating an example technique forevaluating the crosstalk between an INS and an ICD implanted within apatient.

FIG. 21 is a flow diagram illustrating an example technique that may beused to evaluate the extent of the crosstalk between an INS and ICDimplanted within a patient and minimize the crosstalk if the crosstalkexceeds a threshold level.

FIG. 22 is a conceptual illustration of a programmer, which may displayvarious signals indicative of the extent of crosstalk between an ICD andan INS implanted within a patient.

FIG. 23 is a flow diagram of an example technique for categorizingsensed crosstalk between ICD and INS into different categories.

FIG. 24 is a flow diagram of an example technique for extracting datafrom a waveform of an artifact present in an electrical signal sensed byan ICD.

FIG. 25 is a flow diagram illustrating an example technique that may beused to evaluate the extent of the crosstalk between an INS and an ICD.

FIG. 26 is a flow diagram illustrating another example technique thatmay be used to evaluate the extent of the crosstalk between an INS andan ICD.

FIG. 27 is a flow diagram of an example technique for determiningwhether the crosstalk between an ICD and an INS may be adverselyaffecting the impedance measurements taken by the ICD.

FIG. 28 is a flow diagram illustrating an example technique formodifying an electrical stimulation signal generated and delivered by anINS to mitigate the affect on impedance measurements of electrical pathstaken by an ICD.

FIG. 29 is a flow diagram of an example technique for determiningwhether the crosstalk between an ICD and an INS may be adverselyaffecting the impedance measurements taken by the INS.

FIG. 30 is a flow diagram illustrating an example technique forevaluating the integrity of a therapy system.

FIG. 31 is a functional block diagram of an example implantable medicaldevice that includes a neurostimulation module that generates anddelivers electrical stimulation to a tissue site within a patient and acardiac therapy module that generates and delivers electricalstimulation to a heart of the patient.

FIG. 32 is a block diagram illustrating an example system that includesan external device, such as a server, and one or more computing devicesthat are coupled to the INS, ICD, and programmer shown in FIG. 1 via anetwork.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram illustrating an example therapy system 10that provides therapy to patient 12. Therapy system 10 includesimplantable cardiac device (ICD) 16, which is connected to leads 18, 20,and 22, and programmer 24. ICD 16 may be, for example, a device thatprovides cardiac rhythm management therapy to heart 14, and may include,for example, an implantable pacemaker, cardioverter, and/ordefibrillator that provide therapy to heart 14 of patient 12 viaelectrodes coupled to one or more of leads 18, 20, and 22. In someexamples, ICD 16 may deliver pacing pulses, but not cardioversion ordefibrillation pulses, while in other examples, ICD 16 may delivercardioversion or defibrillation pulses, but not pacing pulses. Inaddition, in further examples, ICD 16 may deliver pacing, cardioversion,and defibrillation pulses.

In some examples, ICD 16 may not deliver cardiac rhythm managementtherapy to heart 14, but may instead only sense electrical cardiacsignals of heart 14 and/or other physiological parameters of patient 12(e.g., blood oxygen saturation, blood pressure, temperature, heart rate,respiratory rate, and the like), and store the electrical cardiacsignals and/or other physiological parameters of patient 12 for lateranalysis by a clinician. In such examples, ICD 16 may be referred to asa patient monitoring device. Examples of patient monitoring devicesinclude, but are not limited to, the Reveal Plus Insertable LoopRecorder, which is available from Medtronic, Inc. of Minneapolis, Minn.For ease of description, ICD 16 will be referred to herein as a cardiacrhythm management therapy delivery device.

Therapy system 10 further comprises implantable electrical stimulator26, which is coupled to lead 28. Electrical stimulator 26 may also bereferred to as an implantable neurostimulator (INS) 26. INS 26 may beany suitable implantable medical device (IMD) that includes a signalgenerator that generates electrical stimulation signals that may bedelivered to a tissue site of patient 12, e.g., tissue proximate a vagusnerve, a spinal cord or heart 14 of patient 12.

In some examples, the tissue site may include at least one of anonmyocardial tissue site or a nonvascular cardiac tissue site. Anonmyocardial tissue site may include a tissue site that does notinclude cardiac muscle (e.g., the myocardium). For example, anonmyocardial tissue site may be proximate a muscle other than cardiacmuscle, an organ other than the heart, or neural tissue. A tissue siteproximate a nerve may be a neural tissue site to which delivery ofelectrical stimulation may activate the nerve. In some examples, atissue site proximate a nerve may be in a range of about zerocentimeters to about ten centimeters from the nerve, although otherdistance ranges are contemplated and may depend upon the nerve. Thenonmyocardial tissue site may include extravascular tissue sites orintravascular tissue sites. A nonvascular cardiac tissue site mayinclude, for example, a cardiac fat pad.

In some examples, delivery of electrical stimulation to a tissue siteproximate a nerve or a nonmyocardial tissue site that may not beproximate a nerve may help modulate an autonomic nervous system ofpatient 12. In some examples, INS 26 may deliver electrical stimulationtherapy to a nerve of patient 12 via a lead implanted within vasculature(e.g., a blood vessel) of patient 12. In some examples, INS 26 maydeliver electrical stimulation that is delivered to peripheral nervesthat innervate heart 14, or fat pads on heart 14 that may contain nervebundles. The fat pads may be referred to as a nonvascular cardiac tissuesite.

In the example shown in FIG. 1, electrodes of lead 28 are positionedoutside the vasculature of patient 12 and positioned to deliverelectrical stimulation to a vagus nerve (not shown) of patient 12.Stimulation may be delivered to extravascular tissue sites, for example,when lead 28 is not implanted within vasculature, such as within a vein,artery or heart 14. In other examples, stimulation may be delivered to anonmyocardial tissue site via electrodes of an intravascular lead thatis implanted within vasculature.

In the example shown in FIG. 1, the components of ICD 16 and INS 26 areenclosed in separate housings, such that ICD 16 and INS 26 arephysically separate devices. In other examples, as described withrespect to FIG. 31, the functionality of ICD 16 and INS 26 may beperformed by an IMD that includes both a cardiac therapy module thatgenerates and delivers at least one of pacing, cardioversion ordefibrillation therapy to patient 12 and an electrical stimulationtherapy module that generates and delivers electrical stimulation to atarget tissue site within patient 12, which may be proximate a nerve ormay be an extravascular tissue site that is not proximate a nerve.

Leads 18, 20, 22 extend into the heart 14 of patient 12 to senseelectrical activity of heart 14 and/or deliver electrical stimulation toheart 14. In the example shown in FIG. 1, right ventricular (RV) lead 18extends through one or more veins (not shown), the superior vena cava(not shown), and right atrium 30, and into right ventricle 32. Leftventricular (LV) coronary sinus lead 20 extends through one or moreveins, the vena cava, right atrium 30, and into the coronary sinus 34 toa region adjacent to the free wall of left ventricle 36 of heart 14.Right atrial (RA) lead 22 extends through one or more veins and the venacava, and into the right atrium 30 of heart 14. As described in furtherdetail with reference to FIG. 5, in other examples, an ICD may deliverstimulation therapy to heart 14 by delivering stimulation to anonmyocardial tissue site in addition to or instead of deliveringstimulation via electrodes of intravascular leads 18, 20, 22.

ICD 16 may sense electrical signals attendant to the depolarization andrepolarization of heart 14 via electrodes (not shown in FIG. 1) coupledto at least one of the leads 18, 20, 22. In some examples, ICD 16provides pacing pulses to heart 14 based on the electrical signalssensed within heart 14. These electrical signals sensed within heart 14may also be referred to as cardiac signals or electrical cardiacsignals. The configurations of electrodes used by ICD 16 for sensing andpacing may be unipolar or bipolar. ICD 16 may also providedefibrillation therapy and/or cardioversion therapy via electrodeslocated on at least one of the leads 18, 20, 22. ICD 16 may detectarrhythmia of heart 14, such as fibrillation of ventricles 32 and 36,and deliver defibrillation therapy to heart 14 in the form of electricalpulses. In some examples, ICD 16 may be programmed to deliver aprogression of therapies, e.g., pulses with increasing energy levels,until a fibrillation of heart 14 is stopped. ICD 16 may detectfibrillation employing one or more fibrillation detection techniquesknown in the art.

In the example of FIG. 1, INS 26 has been implanted in patient 12proximate to an nonmyocardial target stimulation site 40, such as atissue site proximate a vagus nerve. For example, INS 26 may besubcutaneously or submuscularly implanted in the body of a patient 12(e.g., in a chest cavity, lower back, lower abdomen, or buttocks ofpatient 12). INS 26 provides a programmable stimulation signal (e.g., inthe form of electrical pulses or a continuous signal) that is deliveredto target stimulation site 40 by implantable medical lead 28, and moreparticularly, via one or more stimulation electrodes carried by lead 28.Proximal end 28A of lead 28 may be both electrically and mechanicallycoupled to connector 42 of INS 26 either directly or indirectly (e.g.,via a lead extension). In particular, conductors disposed in the leadbody may electrically connect stimulation electrodes (and senseelectrodes, if present) of lead 28 to INS 26.

INS 26 may also be referred to as a signal generator. In some examples,lead 28 may also carry one or more sense electrodes to permit INS 26 tosense electrical signals from target stimulation site 40. Furthermore,in some examples, INS 26 may be coupled to two or more leads, e.g., forbilateral or multi-lateral stimulation.

Delivery of electrical stimulation by INS 26 to one or more targettissues sites proximate to a nerve, nerve site, cardiac fat pad, or anextravascular target tissue site that is not proximate a nerve mayprovide cardioprotective benefits to patient 12. An extravascular tissuesite may be outside of heart 14 and outside of arteries, veins, or othervasculature of patient 12. For example, delivery of electricalstimulation to a tissue site proximate a nerve of patient 12 may helptreat heart failure. In addition, delivery of electrical stimulation toa tissue site proximate a nerve of patient 12 to modulate an autonomicnervous system of patient 12 may help reduce or eliminate cardiovascularconditions such as bradycardia, tachycardia, unhealthy cardiaccontractions, ischemia, inefficient heart pumping, inefficientcollateral circulation of heart 14 or cardiac muscle trauma. Delivery ofelectrical stimulation by INS 26 may compliment antitachycardia therapy(e.g., antitachycardia pacing, cardioversion or defibrillation) by ICD16 or provide back-up therapy to the cardiac rhythm therapy provided byICD 16. For example, if ICD 16 is unavailable to provide therapy topatient 12, e.g., due to a low power level, INS 26 may deliver therapyto patient 12 to help terminate or prevent a cardiac event (e.g.,tachycardia).

In some examples, INS 26 delivers electrical stimulation to peripheralnerves that innervate heart 14, or fat pads on heart 14 that may containnerve bundles. In the example shown in FIG. 1, electrodes of lead 28 arepositioned to deliver electrical stimulation to a vagus nerve (notshown) of patient 12. Although INS 26 is referred to throughout theremainder of the disclosure as a “neurostimulator” and as deliveringneurostimulation pulses, in other examples, INS 26 may deliverelectrical stimulation to any suitable nonmyocardial tissue site withinpatient 12, which may or may not be proximate a nerve.

In the example shown in FIG. 1, INS 26 provides electrical stimulationtherapy of a parasympathetic nerve, such as a vagus nerve, of patient12. Stimulation of a parasympathetic nerve of patient 12 may help slowintrinsic rhythms of heart 14, which may facilitate antitachyarrhythmiatherapy (e.g., antitachycardia pacing, cardioversion or defibrillation)delivered by ICD 16. In this way, neurostimulation by INS 26 may helpcontrol a heart rate of patient 12 or otherwise control cardiacfunction.

In other examples, electrodes of lead 28 may be positioned to deliverelectrical stimulation to any other suitable nerve, organ, muscle ormuscle group in patient 12, which may be selected based on, for example,a therapy regimen selected for a particular patient. In some examples,INS 26 may deliver electrical stimulation to other parasympatheticnerves, baroreceptors, the carotid sinus or a cardiac branch of thevagal trunk of patient 12 in order to compliment the delivery of therapyby ICD 16.

The electrical stimulation signals generated and delivered by INS 26 maybe referred to as neurostimulation signals. However, in some examples,INS 26 may deliver electrical stimulation to a target tissue site 40that is not proximate to a nerve. For example, in some examples, INS 26may deliver electrical stimulation to a peripheral nerve field site,whereby electrodes 124 (FIG. 7) are implanted in a region where patient12 experiences pain. The pain may be related to stimulation delivered byICD 16 or a patient condition, such as angina or chronic back pain. Asother examples, INS 26 may deliver electrical stimulation to a muscle,muscle group, organ, or other sites that may not be proximate a nerve.Thus, while “neurostimulation” signals are primarily referred to herein,the disclosure is also applicable to examples in which INS 26 deliverselectrical stimulation to other tissue sites.

As another example, as shown in FIG. 2, INS 26 may be positioned todeliver electrical stimulation to spinal cord 44 of patient 12.Stimulation of spinal cord 44 or nerves branching therefrom by INS 26may help prevent or mitigate occurrences of tachyarrhythmias and mayfacilitate reduction of the level of aggressiveness of the cardiactherapy, such as pacing, cardioversion or defibrillation therapy,delivered by ICD 16. In this way, ICD 16 and INS 26 may operate inconjunction with each other to help prevent arrhythmias of heart 14 ofpatient 12, as well as to terminate detected arrhythmias.

In some examples, depending upon the neurostimulation target, thedelivery of electrical stimulation by INS 26 may also mitigateperceptible discomfort generated from the delivery of pacing pulses orcardioversion/defibrillation shocks by ICD 16. For example, if INS 26delivers electrical stimulation to spinal cord 44 of patient 12, theneurostimulation may produce paresthesia, which may help reduce thediscomfort felt by patient 12 from the delivery of stimulation by ICD16.

In the example shown in FIG. 2, in therapy system 11, INS 26 is coupledto two leads 28, 29 to provide bilateral stimulation of spinal cord 44.Leads 28, 29 may be introduced into spinal cord 44 in the thoracicregion, as shown in FIG. 2. In other examples, leads 28, 29 may beintroduced into spinal cord 44 in the cervical or lumbar regions.Electrodes of leads 28, 29 may be positioned within an intrathecal spaceor epidural space of spinal cord 44, or, in some examples, adjacentnerves that branch off of spinal cord 44. In some examples, leads 28, 29are implanted within patient 12 and positioned such that electrodes ofleads 28, 29 deliver electrical stimulation to locations proximate tothe T1 to T6 thoracic vertebrae of the patient's vertebral column. Forexample, electrodes of at least one of the leads 28, 29 may span the T3to T6 thoracic vertebrae or deliver electrical stimulation to a tissuesite proximate at least one of the T3 to T6 thoracic vertebrae. In otherexamples, leads 28, 29 may be implanted to deliver electricalstimulation to other regions proximate or within spinal cord 44, such asover or near other vertebrae.

In some examples, INS 26 delivers therapy to patient 12 with a voltageamplitude of about 0.2 volts to about 12 volts, a pulse duration ofabout 40 microseconds (μs) to about 600 μs, such as about 50 μs to about500 μs), and a pulse rate of about 1 Hz to about 1 kilohertz (e.g.,about 10 Hz to about 100 Hz). However, other stimulation parametervalues for INS 26 are contemplated. INS 26 may deliver electricalstimulation to patient 12 substantially continuously or periodically. Insome examples, INS 26 may deliver electrical stimulation to patient 12based on the timing of electrical stimulation by ICD 16, such as priorto the delivery of electrical stimulation (e.g., antitachycardia pacingor a defibrillation or cardioversion pulse) by ICD 16, during thedelivery of electrical stimulation by ICD 16, subsequent to the deliveryof electrical stimulation by ICD 16 or any combination of theaforementioned times. In addition, in some examples, INS 26 may deliverelectrical stimulation to patient 12 based on a sensed event or, such asatrial or ventricular depolarization, or based on a sensed physiologicalcondition. The event or physiological condition may be sensed by ICD 16,INS 26 or another sensing device.

ICD 16 and INS 26 may communicate with each other in order for INS 26 totime the delivery of electrical stimulation based on the delivery ofstimulation pulses by ICD 16, where the stimulation pulses may be pacingpulses or cardioversion/defibrillation pulses. ICD 16 and INS 26 maycommunicate directly or indirectly (e.g., via an intermediate device,such as programmer 24) using any suitable communication technique.Examples communication techniques that may be implemented to facilitatecommunication between ICD 16 and INS 26 may include, for example,radiofrequency (RF) communication techniques, optical communicationtechniques, ultrasonic communication techniques, and the like.Communication between ICD 16 and INS 26 may be periodic, e.g., accordingto a regular schedule, or on an as-needed basis, e.g., when INS 26delivers electrical stimulation to patient 12 or when excessivecrosstalk is detected by ICD 16, programmer 24, INS 26 or anotherdevice. Example techniques for evaluating the crosstalk between ICD 16and INS 26 are described below with reference to FIGS. 20, 21, and23-30.

In other examples, INS 26 may deliver electrical stimulation to patient12 independently of the cardiac rhythm therapy delivered by ICD 16. Forexample, INS 26 may be programmed to deliver electrical stimulation topatient 12 according to a schedule that is determined independently ofthe actual delivery of stimulation pulses by ICD 16. The schedule may bedetermined, for example, by a clinician based on a trial stimulationperiod in which multiple therapy schedules for INS 26 are tested onpatient 12. The schedule may dictate when INS 26 actively deliverselectrical stimulation to patient 12 and when INS 26 does not activelydeliver electrical stimulation to patient 12. For example, the schedulemay include a mandatory sleep period for INS 26 during which INS 26reverts to a relatively low-power sleep mode. During the sleep mode, INS26 may not deliver therapy to patient 12 or may deliver a relativelyminimal amount of electrical stimulation therapy to patient 12. Thesleep period may be, for example, when patient 12 is sleeping orotherwise has a relatively low activity level. The sleep period may beuseful for conserving the power source of INS 26.

In some examples, a stimulation schedule for INS 26 may comprise a firstperiod of time in which stimulation is delivered to patient 12substantially continuously or for brief durations (e.g., 0.1 seconds toabout five seconds) and a second period of time during which nostimulation is delivered to patient 12. The first and second periods oftime may be on the order of seconds, minutes, hours or days.

Delivering stimulation to patient 12 via INS 26 periodically rather thansubstantially continuously may help elongate the useful life of therapydelivery by INS 26 or therapy delivery by INS 26 according to aparticular set of stimulation parameter values. Patient 12 may adapt tostimulation provided by INS 26 over time. That is, a certain level ofelectrical stimulation provided to a target tissue site by INS 26 may beless effective over time. This phenomenon may be referred to as“adaptation.” As a result, any beneficial effects to patient 12 from thestimulation delivery by INS 26 may decrease over time. While theelectrical stimulation levels (e.g., amplitude or frequency of theelectrical stimulation signal) may be increased to overcome theadaptation, the increase in stimulation levels may consume more power,and may eventually reach undesirable or harmful levels of stimulation.Adaptation to therapy delivery by INS 26 may be reduced by decreasingthe total amount of stimulation delivered to patient 12 by INS 26, suchas by delivering stimulation to patient 12 when needed (e.g., upon thedetection of an arrhythmia) or according to a schedule in which therapyis turned off or minimized for a period of time. Moreover, noncontinuoustherapy delivery to patient 12 by INS 26 may be more energy efficient.

In addition, delivering stimulation to patient 12 via INS 26periodically rather than substantially continuously may help elongatethe useful life of therapy delivery by INS 26 by extending the life ofthe power source of INS 26. Increasing the amount of time between INS 26recharge or power source replacement may be useful because theinconvenience to patient 12 from the recharge or battery placement maybe minimized.

The values for the therapy parameters that define the electricalstimulation delivered by INS 26 may be organized into a group ofparameter values referred to as a “therapy program” or “therapyparameter set.” “Therapy program” and “therapy parameter set” are usedinterchangeably herein. In the case of electrical stimulation, thetherapy parameters may include an electrode combination, an amplitude,which may be a current or voltage amplitude, a slew rate, and afrequency, and, if INS 26 delivers electrical pulses, a pulse width, anda pulse rate for stimulation signals to be delivered to the patient. Anelectrode combination may include a selected subset of one or moreelectrodes of lead 28, as well as lead 29 if INS 26 is connected to twoleads 28, 29. The electrode combination may also refer to the polaritiesof the electrodes in the selected subset. By selecting particularelectrode combinations, a clinician may target particular anatomicstructures within patient 12. In some cases, INS 25 may deliverstimulation to patient 12 according to a program group that includesmore than one therapy program. The stimulation signals according to thedifferent therapy programs in a therapy group may be delivered on atime-interleaved basis or substantially simultaneously.

The electrical stimulation parameters may also include a duty cycle ofstimulation signals, a timing of the delivery of the electricalstimulation relative to a cardiac cycle of heart 14 of patient 12, and awaveform shape or a signal envelope of the electrical stimulationsignal. A signal envelope may generally traces the outline of theamplitude of a stimulation signal for a given period of time. The signalenvelope may characterize the amplitude ramp-up and ramp-down times,which may be gradual or abrupt.

If INS 26 delivers therapy to patient 12 according to two or moreelectrode combinations, e.g., according to a therapy program groupincluding two or more therapy programs defining at least two differentelectrode combinations, time-interleaving the stimulation signalsdefined each of the therapy programs may result in stimulation that issequentially applied to different electrodes. Varying the tissue site atwhich INS 26 delivers stimulation by delivering therapy according todifferent electrode combinations may also help reduce the patient'sadaptation to therapy delivery by INS 26. For example, sequentiallydelivering stimulation via different electrode combinations may helpreduce the amount of time that a particular tissue site is stimulated.

In some examples, the therapy parameter values with which INS 26generates electrical stimulation therapy for patient 12 may be selectedbased on an effect the stimulation has on heart 14. For example, INS 26may deliver stimulation to a nonmyocardial tissue site within patient 12according to a first therapy program defining values for a set oftherapy parameters, and ICD 16 may assess the response of heart 14 orother portions of the cardiovascular system to the delivery ofstimulation by INS 26. For example, ICD 16 may sense cardiac activityvia electrodes of leads 18, 20, 22. Example responses of heart 14include, for example, proarrhythmic effects. The therapy program may beanalyzed based on a positive or negative response of heart 14 or otherportions of the cardiovascular system to the delivery of stimulation byINS 26. The therapy program may be selected for storage in INS 26, e.g.,for chronic therapy delivery if the test stimulation via the therapyprogram evoked a positive response by heart 14 and/or other portions ofthe patient's cardiovascular system.

Stimulation delivered by INS 26 may have a carryover effect on patient12. A carryover effect generally refers to a physiological effectgenerated in response to the delivery of an electrical stimulationsignal, where the effect persists after termination of the stimulationsignal. The carryover effect may be at least partially attributable to,for example, neurochemicals that are by the patient's body that have anongoing effect on the patient's physiological condition after thetermination of a stimulation signal. Neurochemicals may provide thebenefits of electrical stimulation therapy that continue for a period oftime, such as seconds, minutes, hours or days, after the delivery of astimulation signal by INS 26. If INS 26 delivers electrical stimulationto one or more nerves of patient 12, the carryover effect may also be atleast partially attributable to nerves maintaining a self-stimulatingmode following the delivery of a stimulation signal by INS 26. Nervesmay continue to fire after the termination of a stimulation signal,which may also provide on-going benefits of INS 26 to patient 12 thatcontinue for a period of time, such as seconds, minutes, hours or days,following the termination of a stimulation signal.

In some examples, the stimulation schedule or therapy program (e.g.,frequency of stimulation signals) for INS 26 may be selected based onthe carryover effect of the stimulation delivery by INS 26 on patient12. For example, the interval at which INS 26 delivers stimulationsignals to patient 12 may be substantially equal to or less than aduration of a carryover effect from the delivery of a stimulationsignal. The carryover effect may differ between patients and/or based onthe type of stimulation signals, and, thus, a clinician may test patient12 to determine the duration of a carryover effect. For example, ifdelivery of electrical stimulation therapy by INS 26 causes paresthesiathat patient 12 perceives, the clinician may control INS 26 to deliver astimulation signal and then measure the duration of time required forthe paresthesia to dissipate. This duration of time may be substantiallyequal to a duration of a carryover effect for that particularstimulation signal.

In some cases, ICD 16 may sense electrical noise and interpret theelectrical noise as electrical cardiac signals (e.g., anelectrocardiogram (ECG) or electrogram (EGM) signal). Themisinterpretation of electrical noise may cause ICD 16 to oversensecardiac signals, and, in some cases, erroneously detect an arrhythmia.For example, a processor of ICD 16 may interpret electrical noise as aheart rhythm, and detect the presence of a tachyarrhythmia episode orevent (e.g., a heart cycle measured between successive R-waves that hasa duration less than a threshold value) based on the electrical noise. Atachyarrhythmia episode may include more than one tachyarrhythmia event.Depending on the source of the electrical noise, the electrical noisemay present itself as a relatively fast rhythm, which the processor mayinterpret as one or more tachyarrhythmia events, which may then be usedto detect a tachyarrhythmia episode. ICD 16 may detect the presence of atachyarrhythmia episode by determining whether a certain number ofintervals of a particular number of total intervals have a certainduration, e.g., whether a certain number of intervals are consideredtachyarrhythmia events.

Oversensing heart rhythms may result in inappropriate withholding ordelivery of electrical stimulation to heart 14. For example, oversensingmay cause ICD 16 to detect a tachycardia or fibrillation episode whenheart 14 is in a normal sinus rhythm, which may result in theinappropriate delivery of a high voltage defibrillation shock. Thus,oversensing of heart rhythms by ICD 16 is generally undesirable.

Electrical noise that ICD 16 characterizes as heart rhythms may beattributable to different sources. In some cases, ICD 16 may sense theelectrical stimulation signals (or “neurostimulation signals”) generatedby and delivered to target tissue site 40 by INS 26. The electricalstimulation signals generated by INS 26 and sensed by ICD 16 may bereferred to as “electrical noise” or “interference,” and the presence ofelectrical noise between INS 26 and ICD 16 may be referred to as“crosstalk.” As previously indicated, ICD 16 may control the delivery ofelectrical stimulation to heart 14 based on electrical cardiac signals(e.g., EGM signals) sensed within heart 14. A sensing integrity issuemay arise when ICD 16 senses the electrical stimulation signalsgenerated by INS 26 and mischaracterizes the stimulation signals ascardiac signals. For example, if ICD 16 detects an arrhythmia of heart14 based on electrical signals generated by INS 26 rather than trueelectrical cardiac signals, ICD 16 may unnecessarily deliver electricalstimulation (e.g., pacing pulses or defibrillation/cardioversion shocks)to heart 14.

Therapy system 10 may implement various techniques described herein toreduce the amount of crosstalk between INS 26 and ICD 16. In someexamples, one or more therapy parameter values of the electricalstimulation delivered by INS 26 may be modified in order to minimize thepossibility that the electrical stimulation delivered by INS 26 andsensed by ICD 16 mimics cardiac signals, thereby minimizing thepossibility that ICD 16 mischaracterizes the electrical stimulationdelivered by INS 26 as cardiac signals. Modifying the one or moretherapy parameter values with which INS 26 generates electricalstimulation signals may help modify one or more signal characteristicsof the noise sensed by ICD 16, such as the signal amplitude orfrequency.

As described in further detail below with reference to FIGS. 9-11D, insome examples, if ICD 16 detects an arrhythmia via electrodes of one ormore leads 18, 20, 22 or a housing of ICD 16, ICD 16 may determinewhether the arrhythmia was detected based on noise attributable to theelectrical stimulation delivered by INS 26. ICD 16 may, for example,instruct INS 26 to temporarily stop delivery of electrical stimulationor reduce an intensity of stimulation, and ICD 16 may determine, whileINS 26 is not delivering stimulation or delivering stimulation with alower intensity, whether sensed cardiac signals still indicate anarrhythmia. An intensity of stimulation may be adjusted by modifying oneor more stimulation parameter values, such as the current or voltageamplitude of the stimulation signal, the frequency, slew rate, dutycycle, and, if the stimulation signal comprises stimulation pulses, thepulse width and pulse rate.

If the cardiac signals detected within the suspend period of the INS 26,i.e., the period during which INS 26 does not deliver electricalstimulation or during which INS 26 delivers electrical stimulationhaving a lower intensity, indicate that an arrhythmia is not present,ICD 16 may determine that the arrhythmia was detected based on noisefrom electrical stimulation delivered by INS 26. In some examples, ICD16 may control INS 26 to modify one or more stimulation parameter valuesin order to change the stimulation signal that is detected by ICD 16 andreduce the possibility that ICD 16 senses the stimulation signalsgenerated by INS 26 and mischaracterizes the sensed stimulation signalsas cardiac signals.

As described with reference to FIGS. 12A and 12B, ICD 16 may control INS26 to modify one or more stimulation parameter values by switchingtherapy programs. For example, INS 26 may switch from therapy deliveryaccording to a first therapy program to therapy delivery according to asecond therapy program upon detection of an arrhythmia by ICD 16. Thetherapy programs may define electrical stimulation parameter values withwhich INS 26 may generate electrical stimulation signals. Therapydelivery by INS 26 according to the second therapy program may result inthe generation and delivery of electrical signals that are notmischaracterized by ICD 16 as cardiac signals. For example, the secondtherapy program may define electrical stimulation signals that have awaveform that differs from a cardiac signal in at least one respect,such that ICD 16 does not mischaracterize the electrical stimulationdelivered by INS 26 according to the second therapy program as cardiacsignals. In other examples, INS 26 may switch from therapy deliveryaccording to a first therapy program group to therapy delivery accordingto a second therapy program group upon detection of an arrhythmia by ICD16. The therapy program groups may include one or more therapy programs.

Programmer 24 may include a handheld computing device or a computerworkstation. Programmer 24 may include a user interface that receivesinput from a user. The user interface may include, for example, a keypadand a display, which may for example, be a cathode ray tube (CRT)display, a liquid crystal display (LCD) or light emitting diode (LED)display. The keypad may take the form of an alphanumeric keypad or areduced set of keys associated with particular functions. Programmer 24can additionally or alternatively include a peripheral pointing device,such as a mouse, via which a user may interact with the user interface.In some examples, a display of programmer 24 may include a touch screendisplay, and a user may interact with programmer 24 via the display.

A user, such as a physician, technician, or other clinician, mayinteract with programmer 24 to communicate with ICD 16 and/or INS 26.For example, the user may interact with programmer 24 to retrievephysiological or diagnostic information from ICD 16 and/or INS 26. Auser may also interact with programmer 24 to program ICD 16 and INS 26,e.g., select values for operational parameters of ICD 16 and INS 26,respectively.

For example, the user may use programmer 24 to retrieve information fromICD 16 regarding the rhythm of heart 14, trends therein over time, ortachyarrhythmia episodes. As another example, the user may useprogrammer 24 to retrieve information from ICD 16 regarding other sensedphysiological parameters of patient 12, such as electricaldepolarization/repolarization signals from the heart (referred to as“electrogram” or EGM), intracardiac or intravascular pressure, activity,posture, respiration, heart sounds, or thoracic impedance. As anotherexample, the user may use programmer 24 to retrieve information from ICD16 regarding the performance or integrity of ICD 16 or other componentsof system 10, such as leads 18, 20, and 22, or a power source of ICD 16.

The user may use programmer 24 to program a therapy progression, selectelectrodes used to deliver defibrillation pulses, select waveforms forthe defibrillation pulse, or select or configure a fibrillationdetection algorithm for ICD 16. The user may also use programmer 24 toprogram aspects of other therapies provided by ICD 16, such ascardioversion or pacing therapies. In some examples, the user mayactivate certain features of ICD 16 by entering a single command viaprogrammer 24, such as depression of a single key or combination of keysof a keypad or a single point-and-select action with a pointing device.

The user may also use programmer 24 to retrieve information from INS 26regarding the performance or integrity of INS 26 or leads 28, 29 (if INS26 is connected to more than one lead) or a power source of INS 26. Inaddition, the user may use programmer 24 to program INS 26. For example,with the aid of programmer 24 or another computing device, a user mayselect values for therapy parameters for controlling therapy delivery byINS 26. The values for the therapy parameters may be organized into agroup of parameter values referred to as a “therapy program” or “therapyparameter set.” “Therapy program” and “therapy parameter set” are usedinterchangeably herein.

In the case of electrical stimulation, the therapy parameters for INS 26may include an electrode combination, and an amplitude, which may be acurrent or voltage amplitude, and, if INS 26 delivers electrical pulses,a pulse width, and a pulse rate for stimulation signals to be deliveredto patient 12. An electrode combination may include a selected subset ofone or more electrodes located on implantable lead 28 coupled to INS 26.By selecting particular electrode combinations, a clinician may targetparticular anatomic structures within patient 12. In addition, byselecting values for amplitude, pulse width, and pulse rate, thephysician can attempt to generate an efficacious therapy for patient 12that is delivered via the selected electrode subset.

Programmer 24 may communicate with ICD 16 and INS 26 via wirelesscommunication using any techniques known in the art. Examples ofcommunication techniques may include, for example, low frequency or RFtelemetry, but other techniques are also contemplated. In some examples,programmer 24 may include a programming head that may be placedproximate to the patient's body near the ICD 16 and INS 26 implant sitesin order to improve the quality or security of communication between ICD16 or INS 26, respectively, and programmer 24.

FIG. 3 is a conceptual diagram illustrating ICD 16 and leads 18, 20, 22of therapy system 10 in greater detail. Leads 18, 20, 22 may beelectrically coupled to a stimulation generator, a sensing module, orother modules ICD 16 via connector block 48. In some examples, proximalends of leads 18, 20, 22 may include electrical contacts thatelectrically couple to respective electrical contacts within connectorblock 48. In addition, in some examples, leads 18, 20, 22 may bemechanically coupled to connector block 48 with the aid of set screws,connection pins or another suitable mechanical coupling mechanism.

Each of the leads 18, 20, 22 includes an elongated insulative lead body,which may carry a number of concentric coiled conductors separated fromone another by tubular insulative sheaths. Other lead configurations arealso contemplated, such as configurations that do not include coiledconductors. In the illustrated example, bipolar electrodes 40 and 42 arelocated proximate to a distal end of lead 18. In addition, bipolarelectrodes 54 and 56 are located proximate to a distal end of lead 20and bipolar electrodes 58 and 60 are located proximate to a distal endof lead 22.

Electrodes 50, 54, and 58 may take the form of ring electrodes, andelectrodes 52, 56, and 60 may take the form of extendable helix tipelectrodes mounted retractably within insulative electrode heads 62, 64,and 66, respectively. Each of the electrodes 50, 52, 54, 56, 58, and 60may be electrically coupled to a respective one of the conductors withinthe lead body of its associated lead 18, 20, 22, and thereby coupled torespective ones of the electrical contacts on the proximal end of leads18, 20 and 22.

Electrodes 50, 52, 54, 56, 58, and 60 may sense electrical signalsattendant to the depolarization and repolarization of heart 14. Theelectrical signals are conducted to ICD 16 via the respective leads 18,20, 22. In some examples, ICD 16 also delivers pacing pulses viaelectrodes 50, 52, 54, 56, 58, and 60 to cause depolarization of cardiactissue of heart 14. In some examples, as illustrated in FIG. 2, ICD 16includes one or more housing electrodes, such as housing electrode 68,which may be formed integrally with an outer surface ofhermetically-sealed housing 70 of ICD 16 or otherwise coupled to housing70. In some examples, housing electrode 68 is defined by an uninsulatedportion of an outward facing portion of housing 70 of ICD 16. Divisionsbetween insulated and uninsulated portions of housing 70 may be employedto define two or more housing electrodes. In some examples, housingelectrode 68 comprises substantially all of housing 70. Any of theelectrodes 50, 52, 54, 56, 58, and 60 may be used for unipolar sensingor pacing in combination with housing electrode 68. As described infurther detail with reference to FIG. 6, housing 70 may enclose astimulation generator that generates cardiac pacing pulses anddefibrillation or cardioversion shocks, as well as a sensing module formonitoring the patient's heart rhythm.

Leads 18, 20, 22 also include elongated electrodes 72, 74, 76,respectively, which may take the form of a coil. ICD 16 may deliverdefibrillation pulses to heart 14 via any combination of elongatedelectrodes 72, 74, 76, and housing electrode 68. Electrodes 68, 72, 74,76 may also be used to deliver cardioversion pulses to heart 14.Electrodes 72, 74, 76 may be fabricated from any suitable electricallyconductive material, such as, but not limited to, platinum, platinumalloy or other materials known to be usable in implantabledefibrillation electrodes.

The configurations of therapy system 10 illustrated in FIGS. 1-3 aremerely examples. In other examples, a therapy system may includeepicardial leads and/or patch electrodes instead of or in addition tothe transvenous leads 18, 20, 22 illustrated in FIG. 1. Further, ICD 16and INS 26 need not be implanted within patient 12. In examples in whichICD 16 is not implanted in patient 12, ICD 16 may deliver defibrillationpulses and other therapies to heart 14 via percutaneous leads thatextend through the skin of patient 12 to a variety of positions withinor outside of heart 14 or via external patch electrodes. In examples inwhich INS 26 is not implanted in patient 12, INS 26 may deliverelectrical stimulation to target tissue sites within patient 12 viaexternal electrodes or via percutaneous leads that extend through theskin of patient 12.

In other examples of therapy systems that provide electrical stimulationtherapy to heart 14, a therapy system may include any suitable number ofleads coupled to ICD 16, and each of the leads may extend to anylocation within or proximate to heart 14. Other examples of therapysystems may include three transvenous leads located as illustrated inFIGS. 1-3, and an additional lead located within or proximate to leftatrium 38. Other examples of therapy systems may include a single leadthat extends from ICD 16 into right atrium 30 or right ventricle 32, ortwo leads that extend into a respective one of the right ventricle 32and right atrium 30. An example of this type of therapy system is shownin FIG. 4.

FIG. 4 is a conceptual diagram illustrating another example of therapysystem 78, which includes ICD 16 connected to two leads 18, 22, ratherthan three leads as shown in FIGS. 1-3. Leads 18, 22 are implantedwithin right ventricle 32 and right atrium 30, respectively. Therapysystem 78 shown in FIG. 4 may be useful for providing defibrillation andpacing pulses to heart 14. Therapy system 78 may further include INS 26(not shown in FIG. 4), which is configured to deliver electricalstimulation therapy to modulate an autonomic nervous system of patient12, (e.g., via stimulation of a vagus nerve or within spinal cord 44) inorder to help prevent or mitigate an arrhythmia of patient 12.

FIG. 5 is a conceptual diagram of another example therapy system 80 thatincludes two medical devices to provide therapy to patient 12. Inaddition to INS 26, therapy system 80 includes ICD 82, which deliverselectrical stimulation to heart 14 without intravascular leads. ICD 82is coupled to extravascular leads 83, 84, which each include at leastone electrode 85, 86, respectively. Electrodes 85, 86 may besubcutaneous coil electrodes, which may be positioned within asubcutaneous tissue layer of patient 12. In other examples, electrodes85, 86 may comprise any other suitable type of extravascular electrode.For example, electrodes 85, 86 may include any other type ofsubcutaneous electrode, such as subcutaneous ring electrodes,subcutaneous plate electrodes, subcutaneous patch or pad electrodes, orany other type of extrathoracic electrode, such as a submuscularelectrode, an epicardial electrode or an intramural electrode.

Electrodes 85 may be located within the thoracic cavity of patient 12proximate to right ventricle 32 (FIG. 1), on the patient's side or back,or any other portion of the body appropriate for providing electricalstimulation to heart 14. Electrode 86 may be located within the thoraciccavity of patient 12 proximate left ventricle 36 (FIG. 1) , on thepatient's side or back, or any other portion of the body appropriate forproviding electrical stimulation to the heart. Similar extravascularelectrodes are disclosed in commonly-assigned U.S. Pat. No. 5,261,400 toBardy, which is entitled “DEFIBRILLATOR EMPLOYING TRANSVENOUS ANDSUBCUTANEOUS ELECTRODES AND METHOD OF USE” and issued Nov. 16, 1993, andU.S. Pat. No. 5,292,338 to Bardy, which is entitled “ATRIALDEFIBRILLATOR EMPLOYING TRANSVENOUS AND SUBCUTANEOUS ELECTRODES ANDMETHOD OF USE” and issued Mar. 8, 1994. U.S. Pat. Nos. 5,261,400 and5,292,338 are incorporated herein by reference in their entireties.

Leads 83, 84 may be electrically coupled to stimulation modules, and, insome cases, sensing modules, that are enclosed within housing 87 of ICD82. As with housing 70 of ICD 16 (FIG. 3), housing 87 may comprise ahermetic housing that substantially encloses the components of ICD 16,such as a sensing module, stimulation generator, processor and the like.Components of an example ICD 16 or ICD 82 are described with respect toFIG. 6. ICD 82 may deliver electrical stimulation (e.g., pacing,cardioversion or defibrillation pulses) to heart 14 between electrodes85, 86 e.g., in a bipolar configuration. In other examples, ICD 82 maydeliver electrical stimulation to heart 14 between electrodes 85 andhousing 87 (or an electrode attached to an outer surface of housing 87),or between electrode 86 and housing 87, e.g., in a unipolarconfiguration.

Just as with ICD 16 (FIG. 1) that delivers stimulation to heart 14 viaintravascular electrodes, the delivery of electrical stimulation by INS26 may interfere with the ability of ICD 82 to sense cardiac signals anddeliver appropriate therapy upon the detection of an arrhythmia. ICD 82may include a sensing module similar to that of ICD 16. In some cases,the sensing module may sense the electrical stimulation delivered by INS26 and mischaracterize the signals as cardiac signals, which may causeICD 82 to deliver inappropriate therapy to heart 14 of patient 12.

While the disclosure primarily refers to therapy system 10 including ICD16 (FIG. 1) and INS 26, the description of the techniques, systems, anddevices herein are also applicable to therapy system 80 including ICD 82and INS 26.

FIG. 6 is a functional block diagram of an example configuration of ICD16 (FIG. 1), which includes processor 90, memory 92, stimulationgenerator 94, sensing module 96, telemetry module 98, and power source100. The block diagram shown in FIG. 6 may also illustrate an exampleconfiguration of ICD 82 (FIG. 5). Memory 92 includes computer-readableinstructions that, when executed by processor 90, cause ICD 16 andprocessor 90 to perform various functions attributed to ICD 16 andprocessor 90 herein. Memory 92 may include any volatile, non-volatile,magnetic, optical, or electrical media, such as a random access memory(RAM), read-only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), flash memory, or anyother digital media.

Processor 90 may include any one or more of a microprocessor, acontroller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), orequivalent discrete or integrated logic circuitry. In some examples,processor 90 may include multiple components, such as any combination ofone or more microprocessors, one or more controllers, one or more DSPs,one or more ASICs, or one or more FPGAs, as well as other discrete orintegrated logic circuitry. The functions attributed to processor 90herein may be embodied as software, firmware, hardware or anycombination thereof Processor 90 controls stimulation generator 94 todeliver stimulation therapy to heart 14 according to a selected one ormore of therapy programs, which may be stored in memory 92.Specifically, processor 44 may control stimulation generator 94 todeliver electrical pulses with the amplitudes, pulse widths, frequency,or electrode polarities specified by the selected one or more therapyprograms.

Stimulation generator 94 is electrically coupled to electrodes 50, 52,54, 56, 58, 60, 68, 72, 74, and 76, e.g., via conductors of therespective lead 18, 20, 22, or, in the case of housing electrode 68, viaan electrical conductor disposed within housing 70 of ICD 16.Stimulation generator 94 is configured to generate and deliverelectrical stimulation therapy to heart 14 to manage a rhythm of heart14. For example, stimulation generator 94 may deliver defibrillationshocks to heart 14 via at least two electrodes 68, 72, 74, 76.Stimulation generator 94 may deliver pacing pulses via ring electrodes50, 54, 58 coupled to leads 18, 20, and 22, respectively, helicalelectrodes 52, 56, and 60 of leads 18, 20, and 22, respectively, and/orhousing electrode 68. In some examples, stimulation generator 94delivers pacing, cardioversion or defibrillation therapy in the form ofelectrical pulses. In other examples, stimulation generator 94 maydeliver one or more of these types of therapy in the form of othersignals, such as sine waves, square waves, or other substantiallycontinuous time signals.

In some examples, stimulation generator 94 may include a switch module(not shown in FIG. 6) and processor 90 may use the switch module toselect, e.g., via a data/address bus, which of the available electrodesare used to deliver defibrillation pulses or pacing pulses. The switchmodule may include a switch array, switch matrix, multiplexer, or anyother type of switching device suitable to selectively couplestimulation energy to selected electrodes. In other examples, however,stimulation generator 94 may independently deliver stimulation toelectrodes 50, 52, 54, 56, 58, 60, 68, 72, 74, and 76 or selectivelysense via one or more of electrodes 50, 52, 54, 56, 58, 60, 68, 72, 74,and 76 without a switch matrix.

Sensing module 96 monitors signals from at least one of electrodes 50,52, 54, 56, 58, 60, 68, 72, 74, and 76 in order to monitor electricalactivity of heart 14, e.g., via an EGM signal. Sensing module 96 mayalso include a switch module (not shown in FIG. 6) to select aparticular subset of available electrodes to sense the heart activity.In some examples, processor 90 may select the electrodes that functionas sense electrodes via the switch module within sensing module 96,e.g., by providing signals via a data/address bus. In some examples,sensing module 96 includes one or more sensing channels, each of whichmay comprise an amplifier. In response to the signals from processor 90,the switch module of sensing module 96 may couple the outputs from theselected electrodes to one of the sensing channels.

In some examples, sensing module 96 may include a plurality of channels.One channel of sensing module 96 may include an R-wave amplifier thatreceives signals from electrodes 50 and 52, which are used for pacingand sensing in right ventricle 32 of heart 14. Another channel mayinclude another R-wave amplifier that receives signals from electrodes54 and 56, which are used for pacing and sensing proximate to leftventricle 36 of heart 14. In some examples, in one operating mode ofsensing module 96, the R-wave amplifiers may take the form of anautomatic gain controlled amplifier that provides an adjustable sensingthreshold as a function of the measured R-wave amplitude of the heartrhythm.

In addition, in some examples, one channel of sensing module 96 mayinclude a P-wave amplifier that receives signals from electrodes 58 and60, which are used for pacing and sensing in right atrium 30 of heart14. In some examples, in one operating mode of sensing module 96, theP-wave amplifier may take the form of an automatic gain controlledamplifier that provides an adjustable sensing threshold as a function ofthe measured P-wave amplitude of the heart rhythm. Examples of R-waveand P-wave amplifiers are described in U.S. Pat. No. 5,117,824 to Keimelet al., which issued on Jun. 2, 1992 and is entitled, “APPARATUS FORMONITORING ELECTRICAL PHYSIOLOGIC SIGNALS,” and is incorporated hereinby reference in its entirety. Other amplifiers may also be used.Furthermore, in some examples, one or more of the sensing channels ofsensing module 96 may be selectively coupled to housing electrode 68, orelongated electrodes 72, 74, or 76, with or instead of one or more ofelectrodes 50, 52, 54, 56, 58 or 60, e.g., for unipolar sensing ofR-waves or P-waves in any of chambers 30, 32, or 36 of heart 14.

In some examples, sensing module 96 includes a channel that comprises anamplifier with a relatively wider pass band than the R-wave or P-waveamplifiers. Signals from the selected sensing electrodes that areselected for coupling to this wide-band amplifier may be provided to amultiplexer, and thereafter converted to multi-bit digital signals by ananalog-to-digital converter for storage in memory 92 as an EGM. In someexamples, the storage of such EGMs in memory 92 may be under the controlof a direct memory access circuit. Processor 90 may employ digitalsignal analysis techniques to characterize the digitized signals storedin memory 92 to detect and classify the patient's heart rhythm from theelectrical signals. Processor 90 may detect and classify the heartrhythm of patient 12 by employing any of the numerous signal processingmethodologies known in the art.

If ICD 16 is configured to generate and deliver pacing pulses to heart14, processor 90 may include pacer timing and control module, which maybe embodied as hardware, firmware, software, or any combination thereof.The pacer timing and control module may comprise a dedicated hardwarecircuit, such as an ASIC, separate from other processor 90 components,such as a microprocessor, or a software module executed by a componentof processor 90, which may be a microprocessor or ASIC. The pacer timingand control module may include programmable counters which control thebasic time intervals associated with DDD, VVI, DVI, VDD, AAI, DDI, DDDR,VVIR, DVIR, VDDR, AAIR, DDIR and other modes of single and dual chamberpacing. In the aforementioned pacing modes, “D” may indicate dualchamber, “V” may indicate a ventricle, “I” may indicate inhibited pacing(e.g., no pacing), and “A” may indicate an atrium. The first letter inthe pacing mode may indicate the chamber that is paced, the secondletter may indicate the chamber in which an electrical signal is sensed,and the third letter may indicate the chamber in which the response tosensing is provided. When a pacing code includes “D” as the third letterin the code, it may indicate that the sensed signal is used for trackingpurposes.

Intervals defined by the pacer timing and control module withinprocessor 90 may include atrial and ventricular pacing escape intervals,refractory periods during which sensed P-waves and R-waves areineffective to restart timing of the escape intervals, and the pulsewidths of the pacing pulses. As another example, the pace timing andcontrol module may define a blanking period, and provide signals fromsensing module 96 to blank one or more channels, e.g., amplifiers, for aperiod during and after delivery of electrical stimulation to heart 14.The durations of these intervals may be determined by processor 90 inresponse to stored data in memory 92. The pacer timing and controlmodule of processor 90 may also determine the amplitude of the cardiacpacing pulses.

During pacing, escape interval counters within the pacer timing/controlmodule of processor 90 may be reset upon sensing of R-waves and P-waves.Stimulation generator 94 may include pacer output circuits that arecoupled, e.g., selectively by a switching module, to any combination ofelectrodes 50, 52, 54, 56, 58, 60, 68, 72, 74, and 76 appropriate fordelivery of a bipolar or unipolar pacing pulse to one of the chambers ofheart 14. Processor 90 may reset the escape interval counters upon thegeneration of pacing pulses by stimulation generator 94, and therebycontrol the basic timing of cardiac pacing functions, includinganti-tachyarrhythmia pacing.

The value of the count present in the escape interval counters whenreset by sensed R-waves and P-waves may be used by processor 90 tomeasure the durations of R-R intervals, P-P intervals, P-R intervals andR-P intervals, which are measurements that may be stored in memory 92.Processor 90 may use the count in the interval counters to detect atachyarrhythmia event, such as ventricular fibrillation event orventricular tachycardia event. Upon detecting a threshold number oftachyarrhythmia events, processor 90 may identify the presence of atachyarrhythmia episode, such as a ventricular fibrillation episode, aventricular tachycardia episode, or a non-sustained tachycardia (NST)episode. Examples of tachyarrhythmia episodes that may qualify fordelivery of responsive therapy include a ventricular fibrillationepisode or a ventricular tachyarrhythmia episode. In the case of a NST,however, the count in the interval counters may not meet therequirements for triggering a therapeutic response.

In some examples, processor 90 may operate as an interrupt drivendevice, and is responsive to interrupts from pacer timing and controlmodule, where the interrupts may correspond to the occurrences of sensedP-waves and R-waves and the generation of cardiac pacing pulses. Anynecessary mathematical calculations to be performed by processor 90 andany updating of the values or intervals controlled by the pacer timingand control module of processor 90 may take place following suchinterrupts. A portion of memory 92 may be configured as a plurality ofrecirculating buffers, capable of holding series of measured intervals,which may be analyzed by processor 90 in response to the occurrence of apace or sense interrupt to determine whether heart 14 of patient 12 ispresently exhibiting atrial or ventricular tachyarrhythmia.

In some examples, an arrhythmia detection method may include anysuitable tachyarrhythmia detection algorithms. In one example, processor90 may utilize all or a subset of the rule-based detection methodsdescribed in U.S. Pat. No. 5,545,186 to Olson et al., entitled,“PRIORITIZED RULE BASED METHOD AND APPARATUS FOR DIAGNOSIS AND TREATMENTOF ARRHYTHMIAS,” which issued on Aug. 13, 1996, or in U.S. Pat. No.5,755,736 to Gillberg et al., entitled, “PRIORITIZED RULE BASED METHODAND APPARATUS FOR DIAGNOSIS AND TREATMENT OF ARRHYTHMIAS,” which issuedon May 26, 1998. U.S. Pat. No. 5,545,186 to Olson et al. and U.S. Pat.No. 5,755,736 to Gillberg et al. are incorporated herein by reference intheir entireties. However, other arrhythmia detection methodologies mayalso be employed by processor 90 in other examples.

In the examples described herein, processor 90 may identify the presenceof an atrial or ventricular tachyarrhythmia episode by detecting aseries of tachyarrhythmia events (e.g., R-R or P-P intervals having aduration less than or equal to a threshold) of an average rateindicative of tachyarrhythmia or an unbroken series of short R-R or P-Pintervals. The thresholds for determining the R-R or P-P interval thatindicates a tachyarrhythmia event may be stored within memory 92 of ICD16. In addition, the number of tachyarrhythmia events that are detectedto confirm the presence of a tachyarrhythmia episode may be stored as anumber of intervals to detect (NID) threshold value in memory 92. Insome examples, processor 90 may also identify the presence of thetachyarrhythmia episode by detecting a variability of the intervalsbetween tachycardia events. For example, if the interval betweensuccessive tachyarrhythmia events varies by a particular percentage orthe differences between the coupling intervals are higher than a giventhreshold over a predetermined number of successive cycles, processor 90may determine that the tachyarrhythmia is present.

If processor 90 detects an atrial or ventricular tachyarrhythmia basedon signals from sensing module 96, and an anti-tachyarrhythmia pacingregimen is desired, timing intervals for controlling the generation ofanti-tachyarrhythmia pacing therapies by stimulation generator 94 may beloaded by processor 90 into the pacer timing and control module tocontrol the operation of the escape interval counters therein and todefine refractory periods during which detection of R-waves and P-wavesis ineffective to restart the escape interval counters.

If ICD 16 is configured to generate and deliver defibrillation pulses toheart 14, stimulation generator 94 may include a high voltage chargecircuit and a high voltage output circuit. In the event that generationof a cardioversion or defibrillation pulse is required, processor 90 mayemploy the escape interval counter to control timing of suchcardioversion and defibrillation pulses, as well as associatedrefractory periods. In response to the detection of atrial orventricular fibrillation or tachyarrhythmia requiring a cardioversionpulse, processor 90 may activate a cardioversion/defibrillation controlmodule, which may, like pacer timing and control module, be a hardwarecomponent of processor 90 and/or a firmware or software module executedby one or more hardware components of processor 90. Thecardioversion/defibrillation control module may initiate charging of thehigh voltage capacitors of the high voltage charge circuit ofstimulation generator 94 under control of a high voltage chargingcontrol line.

Processor 90 may monitor the voltage on the high voltage capacitor,e.g., via a voltage charging and potential (VCAP) line. In response tothe voltage on the high voltage capacitor reaching a predetermined valueset by processor 90, processor 90 may generate a logic signal thatterminates charging. Thereafter, timing of the delivery of thedefibrillation or cardioversion pulse by stimulation generator 94 iscontrolled by the cardioversion/defibrillation control module ofprocessor 90. Following delivery of the fibrillation or tachycardiatherapy, processor 90 may return stimulation generator 94 to a cardiacpacing function and await the next successive interrupt due to pacing orthe occurrence of a sensed atrial or ventricular depolarization.

Stimulation generator 94 may deliver cardioversion or defibrillationpulses with the aid of an output circuit that determines whether amonophasic or biphasic pulse is delivered, whether housing electrode 68serves as cathode or anode, and which electrodes are involved indelivery of the cardioversion or defibrillation pulses. Suchfunctionality may be provided by one or more switches or a switchingmodule of stimulation generator 94.

Telemetry module 98 includes any suitable hardware, firmware, softwareor any combination thereof for communicating with another device, suchas INS 26 or programmer 24 (FIG. 1). Under the control of processor 90,telemetry module 98 may receive downlink telemetry from and send uplinktelemetry to programmer 24 with the aid of an antenna, which may beinternal and/or external. Processor 90 may provide the data to beuplinked to programmer 24 and the control signals for the telemetrycircuit within telemetry module 98, e.g., via an address/data bus. Insome examples, telemetry module 98 may provide received data toprocessor 90 via a multiplexer.

In some examples, processor 90 may transmit atrial and ventricular heartsignals (e.g., ECG signals) produced by atrial and ventricular sense ampcircuits within sensing module 96 to programmer 24. Programmer 24 mayinterrogate ICD 16 to receive the heart signals. Processor 90 may storeheart signals within memory 92, and retrieve stored heart signals frommemory 92. Processor 90 may also generate and store marker codesindicative of different cardiac episodes that sensing module 96 detects,and transmit the marker codes to programmer 24. An example pacemakerwith marker-channel capability is described in U.S. Pat. No. 4,374,382to Markowitz, entitled, “MARKER CHANNEL TELEMETRY SYSTEM FOR A MEDICALDEVICE,” which issued on Feb. 15, 1983 and is incorporated herein byreference in its entirety.

The various components of ICD 16 are coupled to power source 100, whichmay include a rechargeable or non-rechargeable battery. Anon-rechargeable battery may be selected to last for several years,while a rechargeable battery may be inductively charged from an externaldevice, e.g., on a daily or weekly basis. Examples of a rechargeablebattery include, but are not limited to, a lithium ion battery, alithium polymer battery or a supercapacitor.

In some examples, data from sensing module 96 may be uploaded to aremote server, from which a clinician or another user may access thedata to determine whether a potential sensing integrity issue exists. Anexample of a remote server includes the CareLink Network, available fromMedtronic, Inc. of Minneapolis, Minn. An example of a system thatincludes an external device, such as a server, and one or more computingdevices that are coupled to ICD 16 and programmer 24 via a network isdescribed below with respect to FIG. 32.

Telemetry module 98 may also be useful for communicating with INS 26,which may also include a telemetry module as described with respect toFIG. 7. In some examples, INS 26 and ICD 16 may communicate with eachother by way of RF communication techniques supported by the respectivetelemetry modules. In addition to or instead of the RF communicationtechniques, INS 26 and ICD 16 may communicate with each other bygenerating electrical communication signals that are sensed via theother device. For example, as described in U.S. Provisional PatentApplication No. 61/110,117 to Burnes et al., which is entitled,“INTERDEVICE IMPEDANCE” and was filed on Oct. 31, 2008, and U.S. patentapplication Ser. No. ______ to Burnes et al. (attorney docket no.P0030861.01/1111-015US01), which is entitled “INTERDEVICE IMPEDANCE” andwas filed on the same date as the present disclosure, in order totransmit information to INS 26, ICD 16 may generate an electrical signalbetween two or more electrodes 50, 52, 54, 56, 58, 60, 68, 72, 74, 76electrically connected to ICD 16, and INS 26 may sense the electricalsignal and retrieve information from the sensed electrical signal. Theelectrical signal may or may not provide therapeutic benefits to patient12. The entire contents of U.S. Provisional Patent Application No.61/110,117 to Burnes et al. and U.S. patent application Ser. No. ______to Burnes et al. (attorney docket no. P0030861.01/1111-015US01) areincorporated herein by reference.

As another example, as described in U.S. patent application Ser. No.______ to Burnes et al. (attorney docket no. P0030861.01/1111-015US01),in order to transmit information to ICD 16, INS 26 may generate anelectrical signal between two or more electrodes 124 electricallyconnected to INS 26 and INS 26 may sense the electrical signal andretrieve information therefrom. Again, the electrical signal may or maynot provide therapeutic benefits to patient 12. In either example, ICD16 or INS 26 may modulate one or more characteristics of the electricalsignal (e.g., an amplitude of frequency of the signal) in order toexchange information with the other device INS 26 or ICD 16,respectively.

Another example of a suitable communication technique for exchanginginformation between ICD 16 and INS 26 is described in commonly-assignedU.S. Pat. No. 4,987,897 to Funke, which is entitled, “BODY BUS MEDICALDEVICE COMMUNICATION SYSTEM,” and issued on Jan. 29, 1991. U.S. Pat. No.4,987,897 to Funke is

FIG. 7 is a functional block diagram of an example INS 26. INS 26includes processor 110, memory 112, stimulation generator 114, switchingmodule 116, telemetry module 118, and power source 120. In the exampleshown in FIG. 7, processor 110, memory 112, stimulation generator 114,switching module 116, telemetry module 118, and power source 120 areenclosed within housing 122, which may be, for example a hermetichousing. As shown in FIG. 7, stimulation generator 114 is coupled tolead 28 either directly or indirectly (e.g., via a lead extension).Alternatively, stimulation generator 114 may be coupled to more than onelead directly or indirectly (e.g., via a lead extension, such as abifurcating lead extension that may electrically and mechanically coupleto two leads) as needed to provide neurostimulation therapy to patient12.

In the example illustrated in FIG. 7, lead 28 includes electrodes124A-124D (collectively referred to as “electrodes 124”). Electrodes 124may comprise ring electrodes. In other examples, electrodes 124 may bearranged in a complex electrode array that includes multiplenon-contiguous electrodes at different angular positions about the outercircumference of lead 28, as well as different levels of electrodesspaced along a longitudinal axis of lead 28. The configuration, type,and number of electrodes 124 illustrated in FIG. 7 are merely exemplary.In other examples, INS 26 may be coupled to any suitable number of leadswith any suitable number and configuration of electrodes. Moreover, lead28 may comprise a shape other than a cylindrical shape. As an example,lead 28 may comprise a paddle-shaped portion that carries electrodes124.

Memory 112 includes computer-readable instructions that, when executedby processor 110, cause INS 26 to perform various functions. Memory 112may include any volatile, non-volatile, magnetic, optical, or electricalmedia, such as a RAM, ROM, NVRAM, EEPROM, flash memory, or any otherdigital media. Memory 112 may store therapy programs, which may bestored in therapy program groups, and operating instructions. Thetherapy programs may define a particular program of therapy in terms ofrespective values for electrical stimulation parameters, such aselectrode combination, electrode polarity, current or voltage amplitude,pulse width and pulse rate. A program group may comprise a plurality oftherapy programs that may be delivered together on an overlapping ornon-overlapping basis. The stored operating instructions may guide thegeneral operation of INS 26 under control of processor 110, and mayinclude instructions for measuring the impedance of electrodes 124.

Stimulation generator 114 generates stimulation signals, which may bepulses as primarily described herein, or continuous signals, such assine waves, for delivery to patient 12 via selected combinations ofelectrodes 124. Processor 110 controls stimulation generator 114according to stored therapy programs and/or program groups in memory 112to apply particular stimulation parameter values specified by one ormore of programs, such as amplitude, pulse width, and pulse rate.Processor 110 may include any one or more microprocessors, controllers,a DSPs, ASICs, FPGAs, or equivalent discrete or integrated digital oranalog logic circuitry, and the functions attributed to processor 110herein may be embodied as software, firmware, hardware or anycombination thereof.

Processor 110 may also control switching module 116 to apply thestimulation signals generated by stimulation generator 114 to selectedcombinations of electrodes 124. In particular, switching module 116couples stimulation signals to selected conductors within lead 28 which,in turn, deliver the stimulation signals across selected electrodes 124.Switching module 116 may be a switch array, switch matrix, multiplexer,or any other type of switching device suitable to selectively couplestimulation energy to selected electrodes. Hence, stimulation generator114 is coupled to electrodes 124 via switching module 116 and conductorswithin lead 28. In some examples, INS 26 does not include switchingmodule 116.

Stimulation generator 114 may be a single or multi-channel stimulationgenerator. In particular, stimulation generator 114 may be capable ofdelivering a single stimulation pulse, multiple stimulation pulses, or acontinuous signal at a given time via a single electrode combination ormultiple stimulation pulses at a given time via multiple electrodecombinations. In some examples, however, stimulation generator 114 andswitching module 116 may be configured to deliver multiple channels on atime-interleaved basis. In this case, switching module 116 serves totime division multiplex the output of stimulation generator 114 acrossdifferent electrode combinations at different times to deliver multipleprograms or channels of stimulation energy to patient 12.

Telemetry module 118 supports wireless communication between INS 26 andan external programmer 24 (FIG. 1) or another computing device, and, insome examples, between INS 26 and ICD 16 under the control of processor110. Processor 110 of INS 26 may receive, as updates to programs, valuesfor various stimulation parameters such as amplitude and electrodecombination, from programmer 24 via telemetry module 118. The updates tothe therapy programs may be stored within memory 112.

The various components of INS 26 are coupled to power source 120, whichmay include a rechargeable or non-rechargeable battery. Anon-rechargeable battery may be selected to last for several years,while a rechargeable battery may be inductively charged from an externaldevice, e.g., on a daily or weekly basis. In other examples, powersource 120 may be powered by proximal inductive interaction with anexternal power source carried by patient 12.

FIG. 8 is block diagram of an example programmer 24. As shown in FIG. 6,programmer 24 includes processor 130, memory 132, user interface 134,telemetry module 136, and power source 138. Programmer 24 may be adedicated hardware device with dedicated software for programming of ICD16 and INS 26. Alternatively, programmer 24 may be an off-the-shelfcomputing device running an application that enables programmer 24 toprogram ICD 16 and INS 26. In some examples, separate programmers may beused to program ICD 16 and INS 26. However, a common programmer 24 thatis configured to program both ICD 16 and INS 26 may provide a morestreamlined programming process for a user, such as a clinician orpatient 12.

A user may use programmer 24 to select therapy programs (e.g., sets ofstimulation parameters), generate new therapy programs, modify therapyprograms through individual or global adjustments or transmit the newprograms to a medical device, such as ICD 16 or INS 26 (FIG. 1). Theclinician may interact with programmer 24 via user interface 134, whichmay include display to present graphical user interface to a user, and akeypad or another mechanism for receiving input from a user.

Processor 130 can take the form one or more microprocessors, DSPs,ASICs, FPGAs, programmable logic circuitry, or the like, and thefunctions attributed to processor 102 herein may be embodied ashardware, firmware, software or any combination thereof. Memory 132 maystore instructions that cause processor 130 to provide the functionalityascribed to programmer 24 herein, and information used by processor 130to provide the functionality ascribed to programmer 24 herein. Memory132 may include any fixed or removable magnetic, optical, or electricalmedia, such as RAM, ROM, CD-ROM, hard or floppy magnetic disks, EEPROM,or the like. Memory 132 may also include a removable memory portion thatmay be used to provide memory updates or increases in memory capacities.A removable memory may also allow patient data to be easily transferredto another computing device, or to be removed before programmer 24 isused to program therapy for another patient. Memory 132 may also storeinformation that controls therapy delivery by ICD 16 and INS 26, such asstimulation parameter values.

Programmer 24 may communicate wirelessly with ICD 16 and INS 24, such asusing RF communication or proximal inductive interaction. This wirelesscommunication is possible through the use of telemetry module 136, whichmay be coupled to an internal antenna or an external antenna. Anexternal antenna that is coupled to programmer 24 may correspond to theprogramming head that may be placed over heart 14, as described abovewith reference to FIG. 1. Telemetry module 136 may be similar totelemetry module 98 of ICD 16 (FIG. 6) or telemetry module 118 of INS 26(FIG. 7).

Telemetry module 136 may also be configured to communicate with anothercomputing device via wireless communication techniques, or directcommunication through a wired connection. Examples of local wirelesscommunication techniques that may be employed to facilitatecommunication between programmer 24 and another computing device includeRF communication according to the 802.11 or Bluetooth specificationsets, infrared communication, e.g., according to the IrDA standard, orother standard or proprietary telemetry protocols. In this manner, otherexternal devices may be capable of communicating with programmer 24without needing to establish a secure wireless connection.

Power source 138 delivers operating power to the components ofprogrammer 24. Power source 138 may include a battery and a powergeneration circuit to produce the operating power. In some examples, thebattery may be rechargeable to allow extended operation. Recharging maybe accomplished by electrically coupling power source 138 to a cradle orplug that is connected to an alternating current (AC) outlet. Inaddition or alternatively, recharging may be accomplished throughproximal inductive interaction between an external charger and aninductive charging coil within programmer 24. In other examples,traditional batteries (e.g., nickel cadmium or lithium ion batteries)may be used. In addition, programmer 24 may be directly coupled to analternating current outlet to power programmer 24. Power source 138 mayinclude circuitry to monitor power remaining within a battery. In thismanner, user interface 134 may provide a current battery level indicatoror low battery level indicator when the battery needs to be replaced orrecharged. In some cases, power source 138 may be capable of estimatingthe remaining time of operation using the current battery.

As previously indicated, in some cases, electrical stimulation signalsgenerated and delivered to patient 12 by INS 26 may be sensed by ICD 16and ICD 16 may mischaracterize the sensed electrical stimulation signalsas cardiac signals. FIG. 9 is a flow diagram illustrating an exampletechnique that therapy system 10 may implement in order to minimize thepossibility that ICD 16 delivers electrical stimulation to heart 14 inresponse to detecting electrical signals generated by INS 26 thatresemble an arrhythmic cardiac signal. While the techniques shown inFIGS. 9-12B, 16, 19-21, and 23-30 are primarily described as beingperformed by one or more of processors 90, 110, 130 of ICD 16, INS 26,and programmer 24, respectively, any one or more parts of the techniquesdescribed herein may be implemented by a processor of one of the devices16, 24, 26, alone or in combination with each other.

INS 26 may deliver neurostimulation to patient 12 (140) and ICD 16 maysense cardiac signals (142). As described above, stimulation generator114 of INS 26 (FIG. 7) may generate electrical stimulation signalsaccording to therapy parameter values defined by a therapy program or atherapy program group, and deliver the signals to patient 12 via aselected subset of electrodes 124 (FIG. 7) of lead 28. ICD 16 may sensecardiac signals of heart 14 via any subset of electrodes 50, 52, 54, 56,58, and 60 of leads 18, 20, 22 (FIGS. 2 and 3) and electrode 68 ofhousing 70. True electrical cardiac signals are generated as heart 14depolarizes and repolarizes.

Processor 90 of ICD 16 may detect a potential arrhythmia based on thesensed cardiac signals (144). The potential arrhythmia may be, forexample, a suspected bradycardia or a suspected tachyarrhythmia. Thecardiac signals sensed by ICD 16 may appear to indicate that heart 14 ofpatient 12 is in an arrhythmia, but, as described herein, ICD 16 maysense noise from delivery of stimulation by INS 26 in addition to thetrue cardiac signals. The noise (also referred to as crosstalk) may maskthe true cardiac activity of heart 14, and, therefore, the detectedarrhythmia may be referred to as a potential arrhythmia.

Processor 90 may implement any suitable technique to detect a potentialarrhythmia of heart 14 (144). Processor 90 of ICD 16 may detect apotential arrhythmia by detecting a threshold number of arrhythmiaevents or an arrhythmia episode, which includes a predetermined numberof arrhythmia events. In some examples, an arrhythmia event may comprisea tachyarrhythmia event, which includes a cardiac cycle that has an R-Rinterval that is less than a predetermined threshold value. If desired,processor 90 may characterize the arrhythmia event as a ventricularfibrillation event, a ventricular tachycardia event or a fastventricular tachycardia event, where different threshold values may beused to characterize the cardiac cycle as the different types of events,e.g., based on the duration of the cardiac cycles. In other examples,the arrhythmia event may comprise a bradycardia event, which includes acardiac cycle that has an R-R interval that is greater than apredetermined threshold.

The threshold duration values for determining whether an R-R intervalqualifies a cardiac cycle as an arrhythmia event may be stored by memory92 of ICD 16 (FIG. 6). In addition, the threshold number of arrhythmiaevents that are characterized as a potential arrhythmia or the number ofarrhythmia events that constitute an arrhythmia episode may be stored bymemory 92 of ICD 16 (FIG. 6) or a memory of another device (e.g., INS 26or programmer 24). In some examples, the threshold number may be abouttwo to about five arrhythmia events, such that processor 90 may detect apotential arrhythmia after about two to about five arrhythmia events aredetected. However, processor 90 may use any suitable threshold number ofarrhythmia events to detect a potential arrhythmia. In other examples,other techniques for detecting a potential arrhythmia may be used.

If processor 90 of ICD 16 does not detect a potential arrhythmia (144),processor 90 may not take any action to modify INS 26 and INS 26 maycontinue delivering electrical stimulation therapy to patient 12 (140)according to the current therapy program or program group. On the otherhand, if processor 90 of ICD 16 detects a potential arrhythmia based onthe sensed cardiac signals (144), processor 90 may determine thatmodification to the neurostimulation signals delivered by INS 26 aredesirable in order to, for example, reduce the crosstalk between ICD 16and INS 26. Thus, processor 90 may initiate the modification to theneurostimulation signals delivered by INS 26 (146).

In some examples, processor 90 of ICD 16 initiates the modification tothe electrical stimulation signals generated and delivered by INS 26.For example, processor 90 of ICD 16 may provide INS 26 with a controlsignal via the respective telemetry modules 98, 118, where the controlsignal causes processor 110 of INS 26 to modify one or more electricalstimulation parameter values of the electrical stimulation generated anddelivered by INS 26. In other examples, processor 90 of ICD 16 maymodify the one or more electrical stimulation parameter values andtransmit the modified parameter values to INS 26. The stimulationparameter values that may be modified include, but are not limited to, acurrent amplitude, a voltage amplitude, a pulse width, a slew rate, apulse rate, a continuous waveform frequency, a duty cycle, an electrodecombination, a timing of the delivery of the electrical stimulationrelative to a cardiac cycle of the heart of the patient, a waveformshape, and a signal envelope of the electrical stimulation signal.

Modifying the one or more electrical stimulation parameter values thatdefine the electrical stimulation signals generated and delivered by INS26 may help change the characteristics of the electrical signaldelivered by INS 26 and sensed by ICD 16. The modified neurostimulationsignal generated and delivered by INS 26 may no longer resemble cardiacsignals, thereby minimizing the possibility that ICD 16 senses theneurostimulation signals and mischaracterizes the signals as cardiacsignals. For example, the modified neurostimulation signal may have afrequency component that falls outside of a sensing bandpass filter usedby ICD 16 to sense cardiac signals. In this way, ICD 16 may “ignore” themodified neurostimulation signals.

The current or voltage amplitude or the frequency of the electricalsignal that ICD 16 senses may change after the electrical stimulationparameter values for INS 26 are modified (146). As an example, if thecurrent or voltage amplitude of the electrical stimulation signalsdelivered by INS 26 is modified, the current or voltage amplitude of themodified neurostimulation signals may no longer resemble cardiac signalsand ICD 16 may no longer sense the electrical stimulation signals ormischaracterize the electrical stimulation signals as cardiac signals.As another example, the current or voltage amplitude of the modifiedneurostimulation signals may below the current or voltage amplitudethreshold used by ICD 16 to identify cardiac signals. As anotherexample, if the frequency of the electrical stimulation signal deliveredby INS 26 is modified, the frequency of the signal sensed by ICD 16 mayno longer have the required frequency or morphology to resemble anarrhythmic cardiac signal (e.g., the neurostimulation signals may nolonger resemble a cardiac signal having short R-R intervals thatcharacterize the signals as ventricular fibrillation cardiac signals).

In some examples, processor 110 of INS 26 may modify the combination ofelectrodes that INS 26 uses to deliver stimulation to patient 12. Thatis, processor 110 may select a different subset of electrodes 124 oflead 28 (FIG. 7) that are activated or modify the polarity of theselected electrodes. Modifying the electrode combination that is used todeliver neurostimulation may help reduce the amount of noise detected byICD 16 from the delivery of electrical stimulation by INS 26. Forexample, modifying the electrode combination with which INS 26 deliverselectrical stimulation signals may help steer the stimulation field awayfrom the sensing field of ICD 16 or at least reduce the amount ofstimulation field that is sensed by ICD 16.

In addition, modifying the electrode combination that is used to deliverneurostimulation may help reduce the amount of noise detected by ICD 16by changing the nature of the noise detected by ICD 16. For example,modifying the neurostimulation electrode combination may change thevector between the electrodes with which the neurostimulation signal isdelivered to tissue of patient 12 and the sensing electrodes of ICD 16that are used to sense a cardiac signal. Changing the relative vectorwith which the sensing electrodes of ICD 16 may sense electrical signalsdelivered by INS 26 may help change the characteristics of theelectrical signals delivered by INS 26 and sensed by ICD 16, such as thecurrent or voltage amplitude of the neurostimulation signals sensed byICD 16, the frequency of the signals, and the like. Other types ofmodifications to the neurostimulation signals generated and delivered byINS 26 are also contemplated.

After modifying the one or more electrical stimulation parameter valuesthat define the electrical stimulation signals generated and deliveredby INS 26 (146), INS 26 may deliver stimulation to patient 12 via themodified electrical stimulation parameter values (147). After INS 26begins delivering stimulation to patient 12 with the modified electricalstimulation signals, processor 90 of ICD 16 may confirm the presence ofthe arrhythmia (148). Processor 90 may confirm the presence of thearrhythmia using any suitable technique, such as the techniques thatwere used to detect the arrhythmia (144). In some examples, if processor90 confirms that the arrhythmia is present, processor 90 of ICD 16 orprocessor 110 of INS 26 may modify the neurostimulation signalsgenerated and delivered by INS 26 at least one more time in an attemptto reduce the electrical noise attributable to 26 (144).

In other examples, if, after modifying the neurostimulation signalsdelivered by INS 26, processor 90 confirms that the arrhythmia ispresent, processor 90 of ICD 16 may determine that the arrhythmia is atrue arrhythmia. In response, processor 90 may characterize a type oftrue arrhythmia detected. For example, based on the R-R interval of thesensed cardiac signals upon which the true arrhythmia was detected,processor 90 may determine whether the arrhythmia is a ventricularfibrillation, a bradycardia event, a supraventricular tachycardia, andthe like. The type of true arrhythmia may be identified in order toselect the appropriate cardiac rhythm therapy. Processor 90 may select atherapy program from memory 92 (FIG. 6) of ICD 16 or a memory of anotherdevice based on the type of true arrhythmia that is detected. Forexample, a plurality of therapy programs for a plurality of differenttypes of arrhythmia may be stored by memory 92. After selecting acardiac rhythm therapy based upon the type of true arrhythmia that isdetected, processor 90 may control stimulation generator 94 (FIG. 6) todeliver electrical stimulation to heart 14 based on the selected therapyin order to terminate the arrhythmia.

In some examples, INS 26 may deliver electrical stimulation therapy topatient 12 according to the modified neurostimulation signals (147) fora finite period of time (rather than substantially indefinitely) andthen revert back to the prior electrical stimulation parameter valuesafter the finite period of time. In some examples, the finite period oftime may be selected by a clinician and stored by memory 92 of ICD 16 ora memory of another device, such as INS 26.

In some examples of the technique shown in FIG. 9, as well as the othertechniques described herein for modifying therapy delivery by INS 26 tominimize crosstalk with ICD 16 (e.g., FIGS. 10 and 11A-11D), INS 26 maydeliver neurostimulation to patient 12 (140) for a test period of time,e.g., for a certain number of cardiac cycles of patient 12, andprocessor 90 of INS 26 may determine if the arrhythmia is detectedduring the delivery of electrical stimulation by INS 26. For example,INS 26 may deliver stimulation to patient 12 for about ten to abouttwenty cardiac cycles (e.g., as indicated by heart beats), and duringthat time, ICD 16 may sense cardiac signals (142) and processor 90 maydetermine whether a potential arrhythmia is detected (144). The testelectrical stimulation delivered by INS 26 may provide therapeuticbenefits to patient 12. In some examples, if the potential arrhythmia isdetected during the delivery of the test neurostimulation to patient 12,processor 90 may determine that the neurostimulation may be interferingwith the detection of true cardiac signals by ICD 16. Thus, in someexamples, processor 90 may initiate the modification to theneurostimulation signals delivered by IND 26 (146), as described abovewith respect to FIG. 9.

FIG. 10 is a flow diagram of an example technique that may beimplemented to determine whether an arrhythmia detected by ICD 16, INS26 or another device may have been attributable to noise fromneurostimulation delivered by INS 26. According to the example techniqueshown in FIG. 10, INS 26 may deliver neurostimulation to a nonmyocardialtissue site (e.g., proximate a nerve) within patient 12 according to atherapy program or therapy program group (140) and ICD 16 may sensecardiac signals (142). Processor 90 of ICD 16 may detect a potentialarrhythmia based on the sensed cardiac signals using any suitabletechnique, such as the techniques described above with respect to FIG. 9(144). If processor 90 does not detect a potential arrhythmia, INS 26may continue delivering neurostimulation to patient 12 according to thetherapy program or therapy program group (140).

If processor 90 of ICD 16 detects a potential arrhythmia (144),processor 90 may adjust the delivery of neurostimulation by INS 26(150). In one example, processor 90 of ICD 16 may generate and deliver acontrol signal to INS 26 via the respective telemetry modules 98, 118.Upon receiving the control signal, processor 110 of INS 26 maytemporarily adjust the delivery of stimulation, such as by suspendingthe active delivery of electrical stimulation to patient 12 or reducingan intensity of a stimulation signal delivered to patient 12. Thecontrol signal may indicate how long INS 26 should deliver therapyaccording to the adjusted parameters or may only indicate that INS 26should adjust the delivery of neurostimulation. For example, the controlsignal may indicate how long INS 26 should suspend the delivery ofneurostimulation. In some examples, processor 110 of INS 26 may refer toinstructions stored within memory 112 of INS 26 that indicate theduration of time for which INS 26 should suspend or otherwise adjust thedelivery of neurostimulation in response to receiving the control signalfrom ICD 16. The stored instructions may also indicate other operatingparameters for the suspension period. For examples, in some cases,rather than deactivating all electrical stimulation signals delivered byINS 26, processor 110 may control stimulation generator 114 to deliverstimulation to patient 12 according to a different set of therapyparameters, such as a therapy program that defines electricalstimulation having a lower intensity (e.g., a lower amplitude orfrequency).

After INS 26 suspends or otherwise adjusts the delivery ofneurostimulation (150), processor 90 of ICD 16 may sense cardiac signalsand determine whether the cardiac signals indicate a potentialarrhythmia (152). If the cardiac signals indicate a potential arrhythmiaafter neurostimulation is suspended or otherwise adjusted, processor 90of ICD 16 may determine that the arrhythmia was not detected based oncrosstalk from the delivery of neurostimulation by INS 26. Processor 90may, for example, determine that the arrhythmia was detected based ontrue cardiac signals and that a true arrhythmia may be present. Thus, inthe technique shown in FIG. 10, processor may generate an arrhythmiaindication if the cardiac signals indicate a potential arrhythmia afterneurostimulation is suspended or otherwise adjusted (154). Thearrhythmia indication may be a value, flag, or signal that is stored ortransmitted to indicate the detection of an arrhythmia.

The arrhythmia indication may be used to control different aspects oftherapy system 10. In some examples, processor 90 may controlstimulation generator 94 (FIG. 6) to generate and deliver at least oneof pacing, cardioversion or defibrillation therapy to heart 14 upon thegeneration of the arrhythmia indication. In other examples, processor 90may confirm the detection of the arrhythmia using physiologicalparameter of patient 12 other than electrical cardiac signals upon thegeneration of the arrhythmia indication. For example, processor 90 mayconfirm the detection of the arrhythmia based on pressure within heart14, as described in U.S. patent application Ser. No. 12/180,160 toMayotte, which is entitled, “SENSING INTEGRITY DETERMINATION BASED ONCARDIOVASULAR PRESSURE,” and was filed on Jul. 25, 2008.

In other examples, processor 90 may confirm the detection of thearrhythmia based on relative tissue perfusion values, blood oxygensaturation levels, blood pressure, heart sounds, cardiovascularpressure, respiratory rate, intrathoracic impedance, cardiac mechanicalactivity, body temperature, acoustic signals indicative of cardiacmechanical activity, and the like. A decrease in tissue perfusion orblood oxygen saturation levels may indicate the presence of anarrhythmia for which therapy delivery to heart 14 is desirable. Forexample, processor 90 may discriminate between hemodynamically toleratedarrhythmias and arrhythmias for which therapy delivery is desirablebased on the blood oxygen saturation level associated with the detectedarrhythmia. Processor 90 may also store the arrhythmia indication inmemory 92 of ICD 16 or a memory of another device, such as programmer 24(FIG. 1) for later analysis by a clinician.

If the cardiac signals sensed by ICD 16 do not indicate a potentialarrhythmia after neurostimulation is suspended or otherwise adjusted,processor 90 of ICD 16 may determine that the previous arrhythmiadetection (144) was based on crosstalk from the delivery ofneurostimulation by INS 26. Accordingly, processor 90 may initiate themodification of the neurostimulation (146), as described with respect toFIG. 9. After the neurostimulation signal is modified, e.g., viamodifying one or more stimulation parameter values, INS 26 may deliverneurostimulation to patient 12 via the modified neurostimulation signaland processor 90 may continue controlling sensing module 96 (FIG. 6) ofICD 16 to sense cardiac signals (142). The technique shown in FIG. 10may then be repeated as necessary.

FIGS. 11A-11D are flow diagrams illustrating a technique that may beimplemented to modify the electrical stimulation signals generated anddelivered by INS 26 in order to reduce the crosstalk between ICD 16 andINS 26. Crosstalk may refer to the phenomenon in which an electricalstimulation signal generated and delivered by INS 26 interferes with theability of ICD 16 to deliver cardiac therapy to heart 14 of patient 12(FIG. 1). For example, the technique shown in FIGS. 11A-11D may be usedto minimize the possibility that ICD 16 detects the neurostimulationsignals and mischaracterizes the signals as cardiac signals by modifyingone or more characteristics of the neurostimulation signal (e.g., thesignal frequency, signal amplitude, slew rate, duty cycle, electrodecombination, waveform shape, signal envelope, pulse width, and thelike).

Processor 90 of ICD 16 may receive cardiac signals sensed via any ofelectrodes 50, 52, 54, 56, 58, 60, 68, 72, 74, and 76 (FIG. 3) of leads18, 20, 22 or housing 70 of ICD 16. Processor 90 may detect a potentialarrhythmia based on the sensed cardiac signals (160). For example, asdescribed above with respect to FIG. 9, processor 90 may detect anarrhythmia event or an arrhythmia episode, which includes apredetermined number of arrhythmia events. In some examples, thearrhythmia event may comprise a tachyarrhythmia event, which includes acardiac cycle that has an R-R interval that is less than a predeterminedthreshold value. The threshold value may be stored within memory 92 ofICD 16 (FIG. 6). In other examples, the arrhythmia event may comprise abradycardia event, which includes a cardiac cycle that has an R-Rinterval that is greater than a predetermined threshold value, which mayalso be stored in memory 92 of ICD 16. The predetermined thresholdnumber of arrhythmia events that processor 90 detects prior todetermining that an arrhythmia episode is detected may be stored inmemory 92 of ICD 16 (FIG. 6) or a memory of another device.

Upon detecting the potential arrhythmia (160), processor 90 of ICD 16may temporarily cause INS 26 to suspend or otherwise adjust the deliveryof neurostimulation signals to patient 12, e.g., by decreasing theintensity of stimulation (150), as described with respect to FIG. 10. Ifprocessor 90 detects the potential arrhythmia after INS 26 suspends orotherwise adjusts the delivery of stimulation signals to patient 12,processor 90 may determine that the sensed cardiac arrhythmia is a truecardiac arrhythmia. Thus, processor 90 may control stimulation generator94 (FIG. 6) of ICD 16 to deliver cardiac therapy to patient 12 in orderto try to terminate the arrhythmia (162). The cardiac therapy may beselected based on the type of arrhythmia that is detected. For example,if processor 90 detects a ventricular fibrillation, processor 90 maycontrol stimulation generator 94 to generate and deliver defibrillationshocks electrical stimulation to heart 14 until the ventricularfibrillation of heart 14 is stopped. In other examples, processor 90 mayconfirm the cardiac arrhythmia based on physiological parameters ofpatient 12 other than sensed electrical cardiac signals, such as basedon vascular pressure, prior to delivering the cardiac therapy toterminate the arrhythmia.

If processor 90 does not detect the potential arrhythmia after INS 26stops actively delivering stimulation signals to patient 12, processor90 may determine that the arrhythmia may have been detected based onelectrical stimulation signals delivered to tissue of patient 12 by INS26. That is, processor 90 may determine that the electrodes 50, 52, 54,56, 58, 60, 68, 72, 74, and 76 (FIG. 3) that were used to sense theelectrical cardiac signals sensed the neurostimulation signals andprocessor 90 mischaracterized the sensed neurostimulation signals aselectrical cardiac signals. This may indicate that the amount ofcrosstalk between ICD 16 and INS 26 exceeds an acceptable amount.Accordingly, either processor 90 of ICD 16 or processor 110 of INS 110may modify a stimulation parameter value used by INS 26 to generate theneurostimulation signals in order to help reduce the amount ofcrosstalk.

As previously indicated, modifying at least one stimulation parametervalue that defines the electrical stimulation therapy provided by INS 26may help change at least one characteristic of the electricalstimulation signal delivered by INS 26, such that ICD 16 either ignoresthe signal (i.e., does not sense the signal) or senses the electricalstimulation signal delivered by INS 26 and recognizes that the sensedsignal is not a true cardiac signal.

In the example shown in FIGS. 11A-11D, processor 110 of INS 26 modifiesone stimulation parameter value at a time. For example, processor 110may modify the stimulation parameter value that least affects theefficacy of therapy delivery to patient 12 by INS 26 and/or most likelyreduces the possibility that ICD 16 will mischaracterize theneurostimulation signal as a cardiac signal. In other examples,processor 110 may modify the stimulation parameter values in any orderor may modify more than one stimulation parameter value at a time. Whilethe description of FIGS. 11A-11D states that processor 110 of INS 26modifies the stimulation parameter values of INS 26, in other examples,processor 90 of ICD 16 or a processor of another device (e.g.,programmer 24) may modify the stimulation parameter values and providethe modified values to processor 110 of INS 26 or processor 110 of INS26 may otherwise act under the direction of processor 90 of ICD 16.

In the example shown in FIG. 11A, processor 110 may modify a stimulationparameter value by modifying a frequency of the neurostimulation signaland processor 110 may subsequently controls stimulation generator 114 ofINS 26 (FIG. 7) to generate and deliver electrical signals having themodified frequency (164). In some examples, processor 110 may store themodified frequency in memory 112 as a therapy program.

In some examples, processor 110 may modify the frequency based on a setof rules that are stored in memory 112 of INS 26 or another device, suchas ICD 16 or programmer 24. The rules may, for example, provide a rangeof frequency values that provide efficacious therapy to patient 12 andthe increments with which processor 110 may modify the frequency (164).The range of frequency values that provide efficacious therapy topatient 12 may indicate the maximum frequency and the minimum frequencyof stimulation signals that provide efficacious therapy to patient 12.Thus, in some examples, the rules may prohibit processor 110 frommodifying the frequency outside of the range of stored frequency valuesin order to prevent processor 110 from modifying the electricalstimulation therapy delivery provided by INS 26 such that the therapydoes not provide therapeutic benefits to patient 12.

In some examples, the rules may indicate the type of modificationprocessor 110 may make to the stimulation parameter based on the type ofarrhythmia that was detected by ICD 16. For example, the rules mayindicate that if a tachyarrhythmia is detected, the frequency of theneurostimulation signal generated by INS 26 should be decreased by aparticular increment. Decreasing the frequency of the neurostimulationsignal may decrease the possibility that ICD 16 will sense thestimulation signal and mischaracterize the neurostimulation signal as acardiac signal. In some examples, decreasing the frequency of aneurostimulation signal may result in electrical noise that does notmeet the requirements of a tachyarrhythmia (e.g., does not appear tohave an R-R interval that is less than a predetermined threshold value).As another example, the rules may indicate that if a bradycardia isdetected, the frequency of the neurostimulation signal generated by INS26 should be increased by a particular increment. The neurostimulationsignal having the increased frequency may no longer resemble a cardiacsignal, or at least may no longer resemble a cardiac signal thatindicates a bradycardia (e.g., does not appear to have an R-R intervalthat is greater than a predetermined threshold value).

After processor 110 of INS 26 modifies the frequency of theneurostimulation signal, processor 90 of ICD 16 may sense cardiacsignals and determine whether an arrhythmia is still detected based onthe sensed cardiac signals (144). If the arrhythmia is no longerdetected, processor 90 of ICD 16 may determine that the prior-detectedarrhythmia was detected based on neurostimulation signals delivered byINS 26 and that the modification to the frequency (164) successfullyreduced the amount of crosstalk between INS 26 and ICD 16. Thus, if thearrhythmia is no longer detected after modifying the frequency of theneurostimulation, processor 110 may not take any further action tomodify the neurostimulation delivered by stimulation generator 114.Stimulation generator 114 may continue generating and deliveringneurostimulation to patient 12 at the modified frequency (166).

On the other hand, if processor 90 of ICD 16 detects a cardiacarrhythmia after the frequency of the neurostimulation signal wasmodified, processor 90 of ICD 16 may control INS 26 to temporarilysuspends or adjusts the delivery of neurostimulation signals to patient12 (150), as described with respect to FIG. 10. If processor 90 detectsthe potential arrhythmia after INS 26 suspends or otherwise adjusts thedelivery of stimulation signals to patient 12, processor 90 maydetermine that the detected arrhythmia was a true arrhythmia and controlstimulation generator 94 (FIG. 6) of ICD 16 to deliver cardiac therapy(e.g., at least one of pacing, cardioversion or defibrillation pulses)to patient 12 in order to try to terminate the arrhythmia (162). Again,in some examples, processor 90 may confirm the presence of thearrhythmia based on a physiological parameter of patient 12 other thanthe electrical cardiac signals prior to delivering the cardiac therapy.

If processor 90 does not detect the arrhythmia after INS 26 stopsdelivering neurostimulation to patient 12, processor 90 may determinethat the arrhythmia was detected based on noise, rather than truecardiac signals. The noise may be at least partially attributable to thecrosstalk from INS 26. Thus, if processor 90 does not detect thearrhythmia after INS 26 stops delivering neurostimulation to patient 12,processor 90 may determine that the prior modification to the frequencyof neurostimulation delivered by INS 26 was insufficient to reduce thenoise and that ICD 16 is still sensing the neurostimulation signals andmischaracterizing the signals as cardiac signals. Accordingly, processor110 of INS 26 may modify at least one more stimulation parameter valuethat defines the neurostimulation therapy delivered by INS 26.

In the example shown in FIG. 11B, processor 110 modifies an amplitude ofthe neurostimulation signal and delivers neurostimulation according tothe modified amplitude (168). In other examples, processor 110 maymodify one or more other types of stimulation parameter values,including the frequency of the neurostimulation signal. The amplitudethat is modified may be a current amplitude or a voltage amplitude andmay depend on, for example, the type of amplitude that is defined by thetherapy program currently implemented by INS 26. In some examples,processor 110 may modify the amplitude of the neurostimulation signal bymodifying the amplitude or pulse width value of the therapy program usedby stimulation generator 114 to generate the neurostimulation signals.In other examples, as described with respect to FIGS. 12A and 12B,processor 110 may select a different therapy program from memory 112 inorder to modify the amplitude.

As previously indicated, modifying an amplitude of the neurostimulationsignal may result in a stimulation signal waveform that differs from acardiac signal. For example, decreasing or increasing the amplitude ofthe neurostimulation signal may result in a signal that falls outside ofthe range of threshold amplitude values that INS 26 uses to detect acardiac signal. Just as with the modification to the frequency (164), insome examples, processor 110 may modify the amplitude of theneurostimulation signal based on a set of rules that are stored inmemory 112 of INS 26. The rules may provide a range of amplitude valuesthat provide efficacious therapy to patient 12 and the increments withwhich processor 110 may modify the neurostimulation signal amplitude(168). In some examples, the rules may control processor 110 to modifythe amplitude values within the stored range of values and preventprocessor 110 from selecting an amplitude value that falls outside ofthe stored range of efficacious amplitude values. In addition, asindicated above, in some examples, the rules may indicate the type ofmodification processor 110 may make to the amplitude based on the typeof arrhythmia that was detected by ICD 16.

After modifying the amplitude of the neurostimulation signal generatedand delivered by INS 26, processor 90 of ICD 16 may sense cardiacsignals and determine whether an arrhythmia is detected (144). If thearrhythmia is no longer detected, processor 90 of ICD 16 may determinethat the prior detected arrhythmia was detected based onneurostimulation signals delivered by INS 26 and sensed by ICD 16, andthat the modification to the neurostimulation signal amplitude (168)successfully changed a characteristic of the neurostimulation signal sothat it no longer resembles a cardiac signal. Thus, if the arrhythmia isno longer detected after modifying the amplitude of the neurostimulationsignal, processor 110 may not take any further action to modify theneurostimulation delivered by stimulation generator 114. Stimulationgenerator 114 may continue generating and delivering neurostimulation topatient 12 at the modified frequency and the modified amplitude (170).

On the other hand, if processor 90 of ICD 16 detects a cardiacarrhythmia after the frequency and amplitude of the neurostimulationsignal were modified, processor 90 of ICD 16 may cause INS 26 totemporarily stop delivering neurostimulation signals to patient 12(150). If processor 90 detects the potential arrhythmia after INS 26suspends or otherwise adjusts the delivery of stimulation signals topatient 12, processor 90 may determine that the detected arrhythmia wasa true arrhythmia and control stimulation generator 94 (FIG. 6) of ICD16 to deliver cardiac therapy to patient 12 in order to try to terminatethe arrhythmia (162).

If processor 90 does not detect the arrhythmia after INS 26 suspends orotherwise adjusts the delivery of neurostimulation to patient 12,processor 90 may determine that the arrhythmia was detected based onnoise (e.g., from INS crosstalk), rather than true cardiac signals.Processor 90 may determine that the prior modifications to the frequencyand amplitude of the neurostimulation signal generated and delivered byINS 26 were insufficient to reduce the crosstalk between ICD 16 and INS26. That is, processor 90 may determine that ICD 16 is still sensing theneurostimulation signals and mischaracterizing the signals as cardiacsignals. Accordingly, processor 110 of INS 26 may modify anotherparameter of the neurostimulation signal, e.g., in response to a controlsignal transmitted to INS 26 from processor 90 of ICD 16. In the exampleshown in FIG. 11B, processor 110 may modify an electrode combinationthat is used to deliver the neurostimulation signal to patient 12 anddeliver neurostimulation with the modified electrode combination (172).The electrode combination may be defined by a therapy program used byINS 26 to generate the neurostimulation signals. Processor 110 maymodify the electrode combination by modifying the therapy programcurrently implemented by INS 26 or by selecting a second therapy programfrom memory 112, whereby the second therapy program defines a differentelectrode combination.

An electrode combination defines the subset of electrodes 124 of lead 28(FIG. 7) coupled to INS 26 that are used to deliver stimulation therapyto patient 12. The electrode combination may also refer to thepolarities of the electrodes in the selected subset. Modifying thesubset of electrodes 124 that are used to deliver stimulation therapy topatient 12 may change the amount of crosstalk between INS 26 and ICD 16by changing the vector between the neurostimulation signal delivered byINS 26 and the electrodes 50, 52, 54, 56, 58, 60, 68, 72, 74, and/or 76(FIG. 3) coupled to ICD 16 that are used to sense cardiac signals. Forexample, delivering the neurostimulation with a different subset ofelectrodes 124 may steer the electrical field generated by the deliveryof neurostimulation to the patient's tissue in a different direction,which may change the intensity of the neurostimulation signal that istransmitted through the patient's body to the sense electrodes of ICD16. This may help reduce the possibility that ICD 16 senses theneurostimulation signal and mischaracterizes the signal as a cardiacsignal. For example, delivering the neurostimulation with a differentsubset of electrodes 124 may change the amplitude or frequency of theneurostimulation signal that is sensed by ICD 16, such that ICD 16 doesnot mischaracterize the neurostimulation signal as a cardiac signal.

Processor 110 may modify the electrode combination by, for example,modifying the quantity of electrodes that are selected to deliverneurostimulation to patient 12, modifying the location of the selectedelectrodes, and/or modifying the spacing between the selectedelectrodes. In addition to or instead of the aforementionedmodifications to the electrode combination, processor 110 may increasethe size of a ground reference electrode area, such as by increasing thenumber of ground electrodes. In some examples, the ground electrode maycomprise an anode electrode, while in other examples the groundelectrode may comprise one or more cathode electrodes. By reducing theresistance of the one or more grounded electrodes, the noise sensed byICD 16 from the neurostimulation may be reduced by reducing the commonmode noise.

In some examples, lead 28 coupled to INS 26 may comprise segmentedelectrodes or partial ring electrodes that do not extend around theentire outer circumference of lead 28. Segmented electrodes may beuseful for directing neurostimulation in a specific direction to enhancetherapy efficacy. In examples in which lead 28 comprises segmented orpartial ring electrodes, processor 110 may modify the electrodecombination by selecting segmented electrodes to deliver theneurostimulation in a different direction, such as a direction away fromICD 16 and its associated electrodes. Processor 110 may modify thedirection of stimulation via the segmented electrodes in order tominimize the far field neurostimulation signal sensed by ICD 16.

In some examples, processor 110 may modify the electrode combinationused to deliver the neurostimulation signal based on a set of rules or apredetermined set of electrode combinations that are stored in memory112 of INS 26. The rules may indicate which electrodes may be activatedor deactivated, and the order in which the activation and deactivationof particular electrodes may take place. For example, the delivery ofstimulation via certain electrodes 124 (FIG. 7) may substantiallyincrease or decrease the efficacy of neurostimulation therapy. Thus, insome examples, the stored rules may indicate that some electrodes shouldnot be deactivated, or at least should be deactivated after otherelectrodes are deactivated, and other electrodes are not preferredelectrodes for delivering electrical stimulation to patient 12.

In some examples, the rules may indicate the type of modificationsprocessor 110 may make to the electrode combination, as well as theorder in which the types of modifications may be made. For example, therules may set forth a hierarchy of modifications, whereby the processor110 may first modify the quantity of selected electrodes, followed bythe selected electrodes, followed by the space between the selectedelectrodes.

After modifying the electrode combination used to deliver aneurostimulation signal by INS 26, processor 90 of ICD 16 may sensecardiac signals and determine whether an arrhythmia is detected (144).If the arrhythmia is no longer detected, processor 90 of ICD 16 maydetermine that the prior detected arrhythmia was detected based onneurostimulation signals delivered by INS 26 and sensed by ICD 16, andthat the modification to the electrode combination (172) successfullyreduced the crosstalk between INS 26 and ICD 16. Thus, if the arrhythmiais no longer detected after modifying the electrode combination used todeliver the neurostimulation signal, processor 110 may not take anyfurther action to modify the neurostimulation delivered by stimulationgenerator 114. Stimulation generator 114 may continue generating anddelivering neurostimulation signals having the modified frequency andamplitude with the modified electrode combination (174).

On the other hand, if processor 90 of ICD 16 detects a cardiacarrhythmia after the electrode combination used to deliver theneurostimulation signal was modified, processor 90 of ICD 16 may causeINS 26 to temporarily stop delivering neurostimulation signals topatient 12 (150). If processor 90 detects the potential arrhythmia afterINS 26 suspends or otherwise adjusts the delivery of stimulation signalsto patient 12, processor 90 may determine that the detected arrhythmiawas a true arrhythmia and control stimulation generator 94 (FIG. 6) ofICD 16 to deliver cardiac therapy to patient 12 in order to try toterminate the arrhythmia (162), or may confirm the arrhythmia based onother physiological parameters of patient 12.

If processor 90 does not detect the arrhythmia after INS 26 suspends orotherwise adjusts the delivery of neurostimulation via the modifiedelectrode combination to patient 12, processor 90 may determine that thearrhythmia was detected based on noise, rather than true cardiacsignals. Processor 90 may determine that the prior modifications to thefrequency and amplitude of the neurostimulation signal and themodification to the electrode combination used to deliver theneurostimulation signals were insufficient to reduce the crosstalkbetween ICD 16 and INS 26. Accordingly, processor 110 of INS 26 maymodify another parameter of the neurostimulation signal. In the exampleshown in FIG. 11C, processor 110 may modify a duty cycle of theneurostimulation, and deliver neurostimulation with the modified dutycycle (176).

A duty cycle of neurostimulation may refer to the proportion of timeduring which a neurostimulation signal is actively delivered to patient12. For example, INS 26 may deliver electrical stimulation to patient 12in a regular duty cycle, whereby the stimulation is delivered for afirst duration of time (e.g., in a single pulse or signal or a burst ofmultiple pulses or signals) and off for a second duration of time,followed by the stimulation delivery for the first duration of time andso forth. The duty cycle may indicate the ratio between the firstduration of time and the total cycle time (the first duration plus thesecond duration of time). Modifying the duty cycle of theneurostimulation may help reduce the duration of the neurostimulation,such that even if crosstalk between ICD 16 and INS 26 is present due tothe delivery of neurostimulation by INS 26, the neurostimulation signalsmay not achieve the required duration of a cardiac signal indicative ofan arrhythmia. That is, as described above, processor 90 of ICD 16 maydetect a potential arrhythmia by detecting a threshold number ofarrhythmia events. If the duration of the neurostimulation is minimizedby modifying the duty cycle of the neurostimulation, theneurostimulation signal may not resemble a cardiac signal comprising thethreshold number of arrhythmia events. Thus, even if theneurostimulation signal resembles an potential arrhythmia event,processor 90 may not detect the threshold number of potential arrhythmiaevents based on the neurostimulation signals.

In some examples, processor 110 may modify the duty cycle of theneurostimulation signals based on a set of rules or by switching toanother therapy program stored in memory 112 of INS 26. The rules mayindicate maximum and minimum duty cycle values for the neurostimulationtherapy, where the maximum and minimum may define a range of duty cyclevalues that may be selected without adversely affecting the efficacy ofneurostimulation therapy. In some examples in which stimulationgenerator 114 delivers electrical stimulation pulses to patient 12,processor 110 may modify the pulse width of the pulses instead of or inaddition to modifying the duty cycle of the neurostimulation.

After modifying the duty cycle of the neurostimulation signal deliveredby INS 26, processor 90 of ICD 16 may sense cardiac signals anddetermine whether an arrhythmia is detected (144). If the arrhythmia isno longer detected, processor 90 of ICD 16 may determine that the priordetected arrhythmia was detected based on neurostimulation signalsdelivered by INS 26 and sensed by ICD 16, and that the modification tothe duty cycle (176) successfully changed a characteristic of theneurostimulation signal so that it no longer resembles a cardiac signal.Thus, if the arrhythmia is no longer detected after modifying theelectrode combination used to deliver the neurostimulation signal,processor 110 may not take any further action to modify theneurostimulation delivered by stimulation generator 114. Stimulationgenerator 114 may continue generating and delivering electricalstimulation signals having the modified frequency, amplitude, and dutycycle, and with the modified electrode combination (178).

On the other hand, if processor 90 of ICD 16 detects a cardiacarrhythmia after the duty cycle of the neurostimulation signal wasmodified, processor 90 of ICD 16 may cause INS 26 to temporarily suspenddelivering neurostimulation signals to patient 12 or otherwise reducethe intensity of stimulation (150). If processor 90 detects thepotential arrhythmia after INS 26 suspends or otherwise adjusts thedelivery of stimulation signals to patient 12, processor 90 maydetermine that the detected arrhythmia was a true arrhythmia and controlstimulation generator 94 (FIG. 6) of ICD 16 to deliver cardiac therapyto patient 12 in order to try to terminate the arrhythmia or may confirmthe arrhythmia based on other physiological parameters of patient 12(162).

If processor 90 does not detect the arrhythmia after INS 26 suspends orotherwise adjusts the delivery of neurostimulation having the modifiedduty cycle, processor 90 may determine that the arrhythmia was detectedbased on noise, rather than true cardiac signals. Processor 90 maydetermine that the prior modification to the frequency, amplitude, andduty cycle of the neurostimulation signal and the modification to theelectrode combination used to deliver the neurostimulation signals wereinsufficient to reduce the crosstalk between ICD 16 and INS 26, suchthat ICD 16 is still sensing the neurostimulation signals andmischaracterizing the signals as cardiac signals. Accordingly, processor110 of INS 26 may modify another therapy parameter of theneurostimulation therapy. In the example shown in FIG. 11D, processor110 may modify the timing between the delivery of neurostimulationsignals relative to the cardiac cycle of heart 14 of patient 12 (180).Processor 110 may then control stimulation generator 114 to deliverneurostimulation signals to patient 12 at the modified times (180).

In some examples, processor 110 controls stimulation generator 114 todeliver neurostimulation signals to patient 12 during a blanking periodof sensing module 96 of ICD 16, and to withhold the delivery ofneurostimulation signals outside of the blanking period. In otherexamples, processor 110 may control stimulation generator 114 to deliverneurostimulation signals to patient 12 during a blanking period ofsensing module 96 and for a relatively short amount of time after theblanking period. The relatively short amount of time may include, forexample, about 1 millisecond (ms) to about 100 ms, although other timeranges are contemplated. The blanking period may refer to a period oftime during which sensing module 96 does not sense any cardiac signals.Therefore, sensing module 96 of ICD 16 may not inadvertently senseneurostimulation signals that are delivered during the blanking period.In some examples, the blanking period may be about 120 ms, althoughother blanking periods are contemplated.

In addition, in some examples, processor 110 may control stimulationgenerator 114 to deliver neurostimulation signals to patient 12 outsideof the blanking period, but relatively early in a cardiac cycle. In someexamples, sensing module 96 (FIG. 6) of ICD 16 may include anautomatically adjusting sense amplifier threshold. In some examples, INS26 may deliver stimulation to patient 12 early in the automaticadjustment period of the sensing module 96 amplifier because the senseamplifier may be less sensitive to noise from delivery ofneurostimulation by INS 26 early in the automatic adjustment period ofthe sensing module 96 amplifier, which may help decrease oversensing.

Processor 110 of INS 26 may time the delivery of neurostimulationsignals during the blanking period of sensing module 96 using anysuitable technique. In some examples, processor 90 of ICD 16 may notifyINS 26 at the beginning of each blanking period, and, in some cases, theend of each blanking period. The notification may be in the form of aflag or another format that may be transmitted to INS 26 via a wired orwireless signal. In other examples, ICD 16 and INS 26 have substantiallysynchronized clocks and memory 112 (FIG. 7) of INS 26 may storeinformation that details the timing of the blanking period of sensingmodule 96. In addition, in some examples, ICD 16 and INS 26 mayperiodically synchronize their respective internal clocks, e.g., by viathe respective telemetry modules 98, 118. For example, ICD 16 mayinstruct INS 26 to synchronize its clock to the clock of ICD 16, or INS26 may instruct ICD 16 to synchronize its clock to the clock of INS 26.Synchronizing clocks may be useful for coordinating stimulationactivity. In some examples, ICD 16 and INS 26 may each include a crystalcontrolled clock, with counters or other means to provide collaborativeclocking or strobe or synchronizing of circuits.

After modifying the timing of neurostimulation such that it is deliveredduring a blanking period of sensing module 96 of ICD 16 (180), processor90 of ICD 16 may sense cardiac signals and determine whether anarrhythmia is detected (144). If the arrhythmia is no longer detected,processor 90 of ICD 16 may determine that the prior detected arrhythmiawas detected based on neurostimulation signals delivered by INS 26 andsensed by ICD 16, and that the modification to the timing of theneurostimulation signal relative to the cardiac signal sufficientlyreduced the crosstalk between INS 26 and ICD 16. Thus, if the arrhythmiais no longer detected after modifying the timing of the delivery of theneurostimulation signals, processor 110 may not take any further actionto modify the neurostimulation delivered by stimulation generator 114.Stimulation generator 114 may continue generating and deliveringneurostimulation to patient 12 via the modified timing (182).

On the other hand, if processor 90 of ICD 16 detects a cardiacarrhythmia after the timing of the neurostimulation delivery wasmodified, processor 90 of ICD 16 may control INS 26 to indefinitelysuspend the delivery of electrical stimulation signals to patient 12 ordeliver electrical stimulation signals according to the adjustedstimulation parameters (184). Either processor 90 of ICD 16 or processor110 of INS 26 may generate an interference indication (186) and transmitthe indication to programmer 24 (FIG. 1) or store the interferenceindication in the respective memory 92, 112. The interference indicationmay indicate that the crosstalk between INS 26 and ICD 16 was notreducible by modifying one or more stimulation parameter values of INS26. The clinician may later retrieve the stored interference indicationand determine whether other measures may be taken in order to reduce thecrosstalk between INS 26 and ICD 16. For example, the clinician maydetermine whether repositioning lead 28 coupled to INS 26 within patient12 may help reduce the crosstalk.

In some examples, other types of therapy parameter values may bemodified in accordance with the technique described with reference toFIGS. 11A-11D. For example, in other examples of the technique shown inFIGS. 11A-11D, processor 110 may modify the waveform shape of theneurostimulation signal, the signal envelope (e.g., by adjusting thestimulation start and stop times), and the like.

For each of the adjustments to the therapy parameter values of INS 26described above with reference to FIGS. 11A-11D, the adjustments may beoccur over several steps, rather than one step as described above. Forexample, the adjustments to the frequency of the neurostimulation signalmay be made in several increments until a predetermined limit isreached. For example, processor 110 of INS 26 may modify the frequencyin 5 Hz increments until the frequency is increased by a total of 50 Hz.Other increment and total limit values are contemplated. In someexamples, a range of parameter values for a particular stimulationparameter may be implemented by processor 110 prior to modifying adifferent type of stimulation parameter value.

In addition, in some examples, two or more therapy parameter values ofINS 26 may be adjusted in a single step, e.g., upon detecting apotential arrhythmia, rather than adjusting independent stimulationparameters in different steps as described above with reference to FIGS.11A-11D. For example, upon detecting a potential arrhythmia, processor110 of INS 26 may modify both the frequency and amplitude of aneurostimulation signal. Other combinations of therapy parameter valuesmay also be modified together.

In some examples, processor 110 of INS 26 may generate electricalstimulation signals according to a different therapy program (or programgroup) in order to modify one or more stimulation parameter values. Forexample, processor 110 may control stimulation generator 114 to generateelectrical stimulation signals according to a first therapy program,and, upon the detection of an arrhythmia, processor 110 may controlstimulation generator 114 to generate electrical stimulation signalsaccording to a second therapy program that has at least one differentstimulation parameter value than the first therapy program. The firstand second therapy programs, as well as any number of other therapyprograms may be stored in memory 112 of INS 26 or a memory of anotherdevice, such as ICD 16.

As previously indicated, if ICD 16 detects an arrhythmia based on theelectrical stimulation signals delivered by INS 26, switching therapyprograms with which INS 26 generates stimulation signals may change thecharacteristics of the neurostimulation signals, which may reduce thepossibility that ICD 16 detects the arrhythmia based on the electricalsignals from INS 26. Thus, in some cases, if ICD 16 detects anarrhythmia after a therapy program of INS 26 is modified, ICD 16 maydetermine that the arrhythmia is a true arrhythmia or at least notdetected based on electrical noise from the delivery of electricalstimulation signals by INS 26.

FIG. 12A is a flow diagram of an operating mode of therapy system 10including ICD 16 and INS 26. Processor 110 of INS 26 may controlstimulation generator 114 to generate and deliver electrical stimulationaccording to a first operating mode to modulate a nerve of patient 12 ordeliver electrical stimulation to a nonmyocardial tissue site of patient12 that is not proximate a nerve (190). In the examples describedherein, the first operating mode is defined by a first therapy program.As previously indicated, a therapy program defines values for thetherapy parameters that define the electrical stimulation delivered byINS 26. In the case of electrical stimulation, the therapy parametersmay include an electrode combination, and an amplitude, which may be acurrent or voltage amplitude, and, if INS 26 delivers electrical pulses,a pulse width for stimulation signals. The therapy program may alsoindicate the timing of the stimulation signals relative to, e.g.,cardiac signals.

ICD 16 may sense cardiac signals via at least one or more of electrodes50, 52, 54, 56, 58, 60, 68, 72, 74, and/or 76 (192). ICD 16 maydetermine whether the sensed cardiac signals, and, in some examples, oneor more other physiological parameter values of patient 12 indicate anarrhythmia of heart 14 (194). If ICD 16 does not detect an arrhythmia(194), processor 110 of INS 26 may continue controlling stimulationgenerator 114 to generate and deliver neurostimulation to patient 12according to the first operating mode. On the other hand, if ICD 16detects an arrhythmia (194), processor 110 of INS 26 may controlstimulation generator 114 of INS 26 to generate and deliver stimulationtherapy according to a second operating mode that is different than thefirst operating mode (196). In the examples described herein, the secondoperating mode is defined by a second therapy program that is differentthan the first therapy program. The second therapy program may compriseat least one stimulation parameter value that differs from the firsttherapy program. In some examples, processor 90 of ICD 16 or anotherdevice (e.g., programmer 24) may instruct processor 110 of INS 26 toswitch operating modes (e.g., switch therapy programs). In addition, insome examples, processor 90 of ICD 16 or another device may transmit thetherapy parameter values of the second operating mode to INS 26.

The therapy parameter values of the first therapy program may beselected to provide patient 12 with efficacious neurostimulationtherapy. In some cases, the therapy parameter values of the firsttherapy program may be selected with little or no regard as to theimpact of the crosstalk from the neurostimulation on the sensing ofcardiac signals by ICD 16. The second therapy program, on the otherhand, may define therapy parameter values that minimize the possibilitythat ICD 16 senses the neurostimulation signals delivered by INS 26 andmischaracterizes the neurostimulation signals as cardiac signals. Forexample, the second therapy program may define a different frequency,current or voltage amplitude, pulse width or duty cycle than the firsttherapy program.

In some examples, the second therapy program defines a stimulationsignal comprising a different waveform than the first therapy program.For example, processor 110 of INS 26 may select a second therapy programthat defines a waveform that has a voltage or current amplitude thatramps up in amplitude and ramps down in amplitude over a longer periodof time than a true cardiac signal (e.g., an EGM signal), such that ICD16 does not mischaracterize the neurostimulation signal as a cardiacsignal. The ramping up and down of a stimulation signal waveform mayhelp reduce the amount of artifact imposed on the signal sensed by ICD16 because the rise time of the neurostimulation signal may be lessabrupt than a rise time of a true cardiac signal. In some examples, thewaveforms defined by the second therapy program may comprisenonrectangular waveforms that gradually ramp up and gradually ramp downin amplitude over time. Example waveforms for stimulation signalsdefined by the second therapy program are shown and described withrespect to FIGS. 13A-13I.

Processor 110 of INS 26 may generate and deliver electrical stimulationsignals according to the second therapy program for a limited period oftime, which may be preset by a clinician or another individual, or maybe based on a sensed physiological parameter of patient 12. For example,processor 110 of INS 26 may generate and deliver electrical stimulationsignals according to the second therapy program until ICD 16 no longerdetects an arrhythmia or a predetermined amount of time following thedetection of an arrhythmia, such as about thirty seconds to about tenminutes following the detection of an arrhythmia. In some examples,processor 110 of INS 26 may generate and deliver electrical stimulationsignals according to the second therapy program until ICD 16 indicatesthat the detected arrhythmia has been terminated. ICD 16 may, forexample, communicate with INS 26 via wireless communication techniques,as previously described.

FIG. 12B is a flow diagram of another example technique that processor110 may implement to control stimulation generator 114 of INS 26. Justas in the technique shown in FIG. 12A, processor 110 may controlstimulation generator 114 to generate and deliver neurostimulation to anonmyocardial tissue site of patient 12 according to a first operatingmode (190). The first operating mode may be characterized by a firsttherapy program that defines a first set of stimulation parameter valueswith which stimulation generator 114 (FIG. 7) of INS 26 generateselectrical stimulation signals. ICD 16 may sense electrical cardiacsignals of patient 12 via at least one or more of electrodes 50, 52, 54,56, 58, 60, 68, 72, 74, and/or 76 and determine whether the sensedelectrical cardiac signals, and, in some examples, one or more otherphysiological parameter values of patient 12 indicate an arrhythmia ofheart 14 (194). If ICD 16 does not detect an arrhythmia (194), processor110 of INS 26 may continue controlling stimulation generator 114 togenerate and deliver neurostimulation to patient 12 according to thefirst operating mode.

On the other hand, if ICD 16 detects a potential arrhythmia (194),processor 110 of INS 26 may control stimulation generator 114 of INS 26to adjust the generation and delivery of electrical stimulation signalsaccording to the first therapy mode (195). Processor 110 may adjust thedelivery of electrical stimulation to patient 12 by suspending thedelivery of stimulation or by decreasing the intensity of stimulation(e.g., modifying an amplitude, frequency, duty cycle, waveform, oranother stimulation parameter). If, upon suspending or otherwiseadjusting the generation and delivery of electrical stimulation signalsaccording to the first therapy mode, processor 110 of INS 26 eitherdetects a potential arrhythmia (e.g., based on sensed physiologicalsignals or by receiving an indication that indicates a potentialarrhythmia is detected) (194), processor 110 may determine that thepotential arrhythmia was not detected based on the electricalstimulation signals from INS 26. Accordingly, processor 110 of INS 26 orprocessor 90 of ICD 16 may generate an arrhythmia indication (154), asdescribed with respect to FIG. 10.

If, upon suspending or otherwise adjusting the generation and deliveryof electrical stimulation signals according to the first operating mode,processor 110 of INS 26 does not detect a potential arrhythmia orreceive an indication that an arrhythmia is detected (194), processor110 may determine that the arrhythmia may have been detected based onnoise resulting from electrical signals delivered by INS 26, rather thantrue cardiac signals. In order to mitigate the crosstalk between INS 26and ICD 16 while still maintaining therapeutic benefits that may beprovided by INS 26, processor 110 may control stimulation generator 114to generate and deliver electrical stimulation signals according to asecond operating mode, e.g., a second therapy program (196).

In some examples, the second operating mode may define a therapy programin which no neurostimulation is delivered to patient 12. Thus, whenprocessor 110 controls stimulation generator 114 to generate and deliverelectrical stimulation signals to patient 12 according to a secondoperating mode, INS 26 may suspend the delivery of stimulation topatient 12.

Processor 110 of INS 26 may control stimulation generator 114 to delivertherapy to patient 12 according to the second operating mode for apredetermined period of time following the switch from the firstoperating mode to the second operating mode. After the period of timehas expired, processor 110 may control stimulation generator 114 toswitch therapy delivery from therapy according to the second operatingmode to therapy according to the first operating mode. The period oftime may be stored in memory 112 of INS 26 or a memory of anotherdevice. The period of time may be selected by a clinician, e.g., basedon how much the clinician wishes to mitigate the possibility ofinadvertent cardiac rhythm therapy by ICD 16. In some examples, theperiod of time with which INS 26 delivers therapy to patient 12according to the second operating mode is in a range of about 100 ms toabout 24 hours or more.

Processor 110 of INS 26 may prohibit further delivery of therapyaccording to the first operating mode (e.g., first therapy program)based upon a number of times therapy delivery by INS 26 is switched fromtherapy according to the first operating mode to therapy according tothe second operating mode. In some examples, processor 110 may prohibitstimulation generator 114 from delivering therapy according to the firstoperating mode if the therapy delivery is switched from the first to thesecond operating modes a threshold number of times within apredetermined period of time. The threshold number of therapy switchesand predetermined period of time may be stored in memory 112 of INS 26or a memory of another device (e.g., ICD 16 or programmer 24).

In the example shown in FIG. 12B, processor 110 may track the number oftimes therapy delivery by INS 26 is switched from therapy according tothe first operating mode to therapy according to the second operatingmode with a counter. For example, upon switching operating modes (e.g.,by switching therapy programs) of INS 26 in response to the detectedarrhythmia, processor 110 of INS 26 may increment a counter (197) anddetermine whether the value of the counter is greater than or equal to athreshold value (198). The value of the counter may indicate the numberof times that processor 110 switched operating modes in response to adetected arrhythmia event. In some examples, the counter may track thenumber of detected arrhythmias for a particular period of time, whichmay be programmed by a clinician and stored in memory 112. After theperiod of time expires, processor 110 may reset the counter.

The threshold value may indicate the number of operating mode switchesthat are acceptable. The threshold value may be stored within memory 112of INS 26 or a memory of another device, such as ICD 16 or programmer24. In some examples, the threshold value may be about two to about ten,such as about three, and a time period for tracking the number ofoperating mode switches may be about one hour to about one day, althoughother threshold values and time periods are contemplated.

In some examples, processor 110 may increment the counter by a numberthat is selected based on the type of arrhythmia that is detected. Forexample, if a ventricular tachyarrhythmia is detected (194), processor110 may increment the counter by a greater number (e.g., two counts)than if a nonsustained tachyarrhythmia is detected. A nonsustainedtachyarrhythmia may comprise fewer arrhythmia events (e.g., R-Rintervals less than a threshold value) than the ventriculartachyarrhythmia. In addition, in some examples, ICD 16 may not delivercardiac rhythm therapy to heart 14 if a nonsustained tachyarrhythmia isdetected, but may deliver therapy if a ventricular tachyarrhythmia isdetected.

If the number of times that processor 110 switched operating modes,i.e., the count, is not greater than or equal to the threshold value,processor 110 may continue delivering therapy according to first andsecond operating modes of INS 26, as described with respect to FIG. 12A.However, if the number of times that processor 110 switched operatingmodes is equal to or exceeds the threshold value, processor 110 maydetermine that the delivery of electrical stimulation according to thefirst operating mode results in excessive interference with the properdetection of cardiac signals by ICD 16. Thus, if the number of timesthat processor 110 switched operating modes is greater than or equal tothe threshold value, processor 110 may prohibit any further delivery ofelectrical stimulation signals generated according to the first therapyprogram (199). That is, processor 110 may indefinitely switch to thesecond operating mode of INS 26. For example, processor 110 may controlstimulation generator 114 to generate and deliver electrical stimulationtherapy to patient 12 according to a second therapy programindefinitely, rather than continuing to switch between first and secondtherapy programs.

Processor 110 may prohibit the generation and delivery of electricalstimulation according to the first operating mode until userintervention is received, e.g., to assess the extent of crosstalk. Theuser intervention may comprise, for example, input from patient 12 orthe clinician resetting the counter, such that INS 26 may deliverstimulation signals that are generated in accordance with the firsttherapy program. The input may be received via user interface 134 (FIG.8) of programmer 24 or a user interface of another computing device,which may transmit the user input to processor 110 via the respectivetelemetry modules 136 (FIG. 8), 118 (FIG. 7). In addition, in someexamples, the user input may be received from a clinician at a remotelocation, e.g., via the system including a network that is describedwith respect to FIG. 32.

In some cases, processor 110 may generate an interference indicationthat is transmitted to patient 12 or a clinician, e.g., via programmer24. For example, processor 110 may transmit the interference indicationto programmer 24 via telemetry module 118 (FIG. 7) and programmer 24 mayreceive the indication via telemetry module 136 (FIG. 8) and generate aninterference indication to notify patient 12 or another person thatclinician intervention may be necessary to mitigate crosstalk betweenICD 16 and INS 26. Processor 110 may generate the interferenceindication in response to the mode switch counter value exceeding thethreshold.

FIGS. 13A-13I are conceptual illustrations of example non-rectangularwaveforms that may be defined by a second operating mode implemented byINS 26 to generate neurostimulation signals after the detection of anarrhythmia. FIG. 13A illustrates a ramped square waveform 200, whichincludes a plurality of waves 202. Each wave 202 includes a leading edge204 that gradually increases in amplitude over time and a trailing edge205 that follows the leading ledge 204 and gradually decreases in time.In some examples, leading edge 204 exhibits a substantially continuousincrease in amplitude, such that leading edge 204 has a differentamplitude at subsequent points in time. Similarly, trailing edge 205 mayexhibit a substantially continuous decrease in amplitude, such thattrailing edge 205 has a different amplitude at subsequent points intime. Although leading edge 204 and trailing edge 205 are illustrated ashaving substantially equal, but opposite slopes, in other examples,leading edge 204 and trailing edge 205 may have slopes of differentmagnitude.

In some examples, stimulation generator 114 of INS 26 may generate theramped square wave by generating a square wave stimulation signal andmodulating the amplitude by a relatively slow sine wave. For example,stimulation generator 114 may generate square wave signals having afrequency of about 80 Hz and a pulse duration of about 300 μs durationpulses, and modulate the amplitude of the square wave from about 0% toabout 100% by an approximately 3 Hz sine wave. The resulting square wavesignal may have a frequency of about 80 Hz and a signal envelope ofabout 3 Hz.

FIG. 13B illustrates a stair step square waveform 206, which includes aplurality of waves 207. Each wave 207 includes a leading edge 208 and atrailing edge 209. The lead edge 208 includes stepwise increases inamplitude over time, whereas the trailing edge 209 includes stepwisedecreases in amplitude over time. Although FIG. 13B illustrates waves207 in which leading edge 208 and trailing edge 209 increase anddecrease, respectively, in substantially equal increments of amplitude,in other examples each step of leading edge 208 and trailing edge 209may increase and decrease, respectively, in amplitude by differentmagnitudes. Moreover, the rising edge of each step in leading edge 208may have a different absolute magnitude than other steps in leading edge208, such that some steps of leading edge 208 are larger than others.Similarly, each step of trailing edge 209 may have a different absolutemagnitude than other steps in trailing edge 209.

FIG. 13C illustrates rounded square waveform 210, which includes aplurality of waves 211. Each wave 211 defines a leading edge 204 thatgradually increases in amplitude over time and a trailing edge 205 thatgradually decreases in time, as described with respect to ramped squarewaveform 200 in FIG. 13A. In addition, waves 211 of rounded squarewaveform 210 includes rounded portion 212 between leading edge 204 andtrailing edge 205. Rounded portion 212 may help further distinguishneurostimulation waveform 210 from a sinus rhythm of heart 14 (FIG. 1)because of the gradual increase and decrease in amplitude. In contrast,the sinus rhythm of heart 14 may exhibit a sharper increase and decreasein amplitude.

In some examples, stimulation generator 114 may generate rounded squarewaveform 210 shown in FIG. 13C by passing a square wave signal or asubstantially square wave signal through a resistor-capacitor (RC) lowpass filter with a cutoff frequency in a range of about 20 Hz to about100 Hz, such as about 60 Hz. The RC low pass filter may help eliminatethe relatively rapid rise time of the square wave, which may help reducethe stimulation signal artifact imposed on ICD 16 because the resultingrounded square wave may no longer resemble a true electrical cardiacsignal, which may comprise a relatively rapid rise time.

FIG. 13D illustrates trapezoidal waveform 214, which includes aplurality of waves 215 comprising a substantially trapezoidal shape.Each trapezoidal wave 215 comprises leading edge 216 and trailing edge217, which follows leading edge 216 in time. Leading edge 216 maycomprise a greater slope compared to lead edge 204 of ramped square wave202 (FIG. 13A). Similarly, trailing edge 217 may comprise a smallerslope (or a greater absolute slope value) compared to trailing edge 205of ramped square wave 202 (FIG. 13A). Although leading edge 216 andtrailing edge 217 are illustrated as having substantially equal, butopposite slopes, such that the waves 215 define isosceles trapezoids, inother examples, leading edge 216 and trailing edge 217 may have slopesof different magnitude.

FIG. 13E illustrates triangular waveform 218, which includes a pluralityof waves 219 defining a substantially triangular shape. Waves 219 eachcomprise leading edge 220 and trailing edge 221, which follows leadingedge 220 in time. Leading edge 220 and trailing edge 221 of each wave219 may have slopes of substantially equal magnitude, or may havedifferent slopes. In some examples, upon the detection of an arrhythmia,processor 110 of INS 26 may control stimulation generator 114 togenerate and deliver electrical stimulation signals comprising astair-step triangular waveform. Just as with the stair step square waveshown in FIG. 13B, a leading edge of the stair step triangular wave maydefine stepwise increases in amplitude over time, and a trailing edge ofthe waveform may define stepwise decreases in amplitude over time.

In some examples, stimulation generator 114 of INS 26 may generate anddeliver biphasic stimulation signals, as shown in FIG. 13F. FIG. 13Fillustrates biphasic triangular waveform 222, which includes triangularwaves 219 having a positive amplitude and triangular waves 223 having anegative amplitude. Biphasic triangular waveform 222 may includealternating positive amplitude triangular waves 219 and negativeamplitude triangular waves 223. Biphasic waveforms may also helpdistinguish neurostimulation signals from cardiac signals. Other typesof biphasic waveforms are also contemplated, such as biphasic squarewaves.

Stimulation generator 114 of INS 26 may also generate and deliverneurostimulation to patient 12 via a sine waveform. FIG. 13G illustratessine waveform 224, which includes a periodic wave 225 defined by a sinefunction. In some examples, as shown in FIG. 13H, stimulation generator114 may also generate and deliver neurostimulation signals having a halfsine waveform 226, such as the positive half of a sine wave or arectified sine wave. The half sine wave may have a duration ofapproximately 200 microseconds (μs), although other signal durations arecontemplated. In other examples, stimulation generator 114 of INS 26 maygenerate and deliver neurostimulation signals comprising a stepwise halfsine waveform 228, as shown in FIG. 13I

As previously described with respect to biphasic triangular waveform 222in FIG. 13F, in some examples, the second therapy program implemented byprocessor 110 of INS 26 after the detection of an arrhythmia may definea biphasic signal. That is, processor 110 may control stimulationgenerator 114 to deliver stimulation signals to selected electrodes 124(FIG. 7) of lead 28 such that the selected electrodes reverse polaritywith each subsequent pulse or, in examples in which continuous wavesignals are delivered, each subsequent half wave. FIGS. 14A and 14Bprovide a conceptual illustration of a configuration of electrodepolarities that may be employed in order to for INS 26 to deliver abiphasic neurostimulation signal to patient 12 in the second operatingmode. FIGS. 13F and 13G illustrate examples of biphasic waveforms thatmay be generated and delivered to patient 12.

FIGS. 14A and 14B illustrate lead 232 comprising a plurality ofelectrodes 234A-234H, which may comprise ring electrodes, partial ringelectrodes or segmented electrodes that extend around less than the fullouter perimeter of lead 232. In the example shown in FIGS. 14A and 14B,lead 232 may comprise a cylindrical lead body with a circularcross-section (when the cross-section is take in a directionsubstantially orthogonal to a longitudinal axis of lead 232). Lead 232may be coupled to stimulation generator 114 of INS 26 instead of or inaddition to lead 28 and/or lead 29 (FIG. 2). Although eight electrodes234A-234H are shown in FIGS. 14A and 14B, in other examples, lead 232may comprise any suitable number of electrodes, which may be greaterthan or fewer than eight.

FIG. 14A illustrates a first example electrode combination that may bedefined by a second therapy program that processor 110 of INS 26 mayimplement upon the detection of an arrhythmia. In the electrodeconfiguration shown in FIG. 14A, electrodes 234A-234D are selected to beanodes and electrodes 234E-234H are selected to be cathodes. Stimulationgenerator 114 may generate a first stimulation pulse or another type ofstimulation signal and transmit the stimulation pulse or signal toelectrodes 234A-234H via the conductors within lead 232. An electricalfield may be generated through the patient's tissue as the electricalsignal flows between the anode electrodes 234A-234D and the cathodeelectrodes 234E-234H. In other examples, a subset of electrodes234A-234H may be selected as part of the electrode combination.

FIG. 14B illustrates a second electrode combination defined by thesecond therapy program in which electrodes 234A-234D are selected to becathodes and electrodes 234E-234H are selected to be anodes. Thus,compared to the first electrode combination shown in FIG. 14A,electrodes 234A-234H have reversed polarity. Stimulation generator 114may utilize the electrode combination shown in FIG. 14B to deliver asubsequent stimulation pulse or wave, i.e., subsequent to the pulse orwave delivered with the electrode combination shown in FIG. 14A. Anelectrical field may be generated through the patient's tissue as theelectrical signal flows between the anode electrodes 234E-234H and thecathode electrodes 234A-234D.

In accordance with an example of the second operating mode of INS 26,stimulation generator 114, e.g., with the aid of switching module 116(FIG. 7) may continue delivering alternating pulses with the electrodecombinations shown in FIGS. 14A and 14B. In some examples, stimulationgenerator 114 may deliver neurostimulation to patient 12 with the sameelectrode combination (e.g., the same polarity configuration) for two ormore pulses or stimulation waves in a row and subsequently deliverneurostimulation to patient 12 to an electrode combination havingreversed polarities. For example, in other examples, stimulationgenerator 114 may deliver two or more pulses with the electrodecombination shown in FIG. 14A followed by two or more pulses with theelectrode combination shown in FIG. 14B. In addition, in other examples,INS 26 may deliver a biphasic neurostimulation signal to patient 12using the electrodes of two or more leads, rather than one lead as shownin FIGS. 14A and 14B.

Delivering neurostimulation signals to patient 12 via a biphasic signal(e.g., via electrode combinations with alternating polarity) may helpreduce the neurostimulation artifact impact on ICD 16 or anotherphysiological parameter monitoring device. The stimulation output netenergy artifact effect sensed by ICD 16 may be approximately zero due tothe rapid encounter of alternate polarity artifact that may cancel outthe neurostimulation signal. In addition, delivering neurostimulationsignals to patient 12 via a biphasic signal may help limit the bandwidthof the transmitted neurostimulation signal, and limiting the bandwidthmay help increase the possibility that ICD 16 may filter out theneurostimulation signal, e.g., via a bandpass filter. Further, in someexamples, sensing module 98 of ICD 16 may be configured to disregard orattenuate the alternating polarity neurostimulation signals. Thus, ifINS 26 delivers biphasic neurostimulation signals, ICD 16 may not sensethe neurostimulation signals and if ICD 16 senses the neurostimulationsignals, ICD 16 may not mischaracterize the neurostimulation signals ascardiac signals.

In either or both the first and second operating modes of INS 26, INS 26may deliver electrical stimulation signals to patient 12 with anelectrode combination that reduces the extent of the energy and/orelectrical field that leaves the target tissue site 40, thereby reducingthe intensity of neurostimulation signal that traverses through thepatient's body and is sensed by ICD 16. The anodes and cathodes of theelectrode combination may be selected such that the stimulation fieldgenerated by the delivery of neurostimulation via the anodes andcathodes (i.e., the selected electrodes) may be relatively focusedwithin target tissue site 40 (FIG. 1).

FIGS. 15A-15F illustrate different examples of electrode combinationsthat may be used to deliver neurostimulation therapy to patient 12. Inthe electrode combinations shown in FIGS. 15A-15F, the anodes andcathodes of the electrode combination are positioned relative to eachother to help reduce the extent of the size of the electrical field (orstimulation field) that is generated as a result of the delivery ofneurostimulation signals by INS 26. In some examples, the electrodecombinations shown in FIGS. 15A-15F may be used to deliver a pluralityof stimulation pulses with an interval of time between each pulse, or aplurality of bursts of electrical stimulation that are separated by aninterval of time, where each burst includes a plurality of stimulationpulses.

In some examples, during a programming session in which a clinicianselects the one or more electrode combinations for a second operatingmode of INS 26, the clinician may utilize a user interface thatgraphically represents the stimulation field generated by stimulationdelivery with a particular subset of electrodes of the one or more leadscoupled to INS 26. An example of a user interface that may be used toselect an electrode combination for the delivery of neurostimulation isdescribed in commonly-assigned U.S. patent application Ser. No.11/999,722 to Goetz et al., entitled, “USER INTERFACE WITH TOOLBAR FORPROGRAMMING ELECTRICAL STIMULATION THERAPY,” which was filed on Dec. 6,2007 and is incorporated herein by reference in its entirety.

As described in U.S. patent application Ser. No. 11/999,722 to Goetz etal., a user interface may display a representation of implantedelectrical leads in conjunction with at least one menu with icons thatthe user can use to adjust the stimulation field of the stimulationtherapy with one or more field shape groups. For example, one menu maybe a field shape selection menu that provides field shapes to indicatethe resulting stimulation field according to initial stimulationparameters. Another menu may be a manipulation tool menu that allows auser to perform certain actions on the field shapes to adjust thestimulation therapy. The user interface may be useful for selecting anelectrode combination and other stimulation parameter values that focusthe stimulation field within target tissue site 40 (FIG. 1). Focusingthe neurostimulation within the desired target tissue site 40 may helpminimize the extent of the stimulation field that falls outside oftarget tissue site 40, particularly in a direction towards heart 14(FIG. 1), and decrease the extent to which the stimulation field may besensed by ICD 16.

FIG. 15A illustrates an example of a guarded cathode electrodecombination 236 that may be selected during the second operating mode ofINS 26 in order to help focus the neurostimulation delivered to patient12. In a guarded cathode arrangement, two or more anodes are positionedaround a cathode of the electrode combination. In FIG. 15A, electrode234D of lead 232 is selected as a cathode of the electrode combinationand electrodes 234C and 234E are selected as anodes. In the exampleshown in FIG. 15A, the anode electrodes 234C, 234E and cathode electrode234D are substantially linearly aligned along a longitudinal axis oflead 232. The anode electrodes 234C and 234E surrounding the cathodeelectrode 234D may be useful for focusing a stimulation field generatedby the delivery of electrical stimulation via electrode combination 236.In particular positioning anode electrodes 234C and 234E on oppositesides of cathode electrode 235D may help limit the size of thestimulation field resulting from the delivery of stimulation via theelectrode combination 236.

In some examples, INS 26 may be coupled to two or more leads, directlyor via one or more lead extensions, such as a bifurcated lead extension.FIG. 15B illustrates a configuration in which INS 26 is coupled to lead232 including eight electrodes 234A-234H, lead 240 including fourelectrodes 242A-242D, and lead 244 including four electrodes 246A-246D.Electrodes 232A-232H, 242A-242D, 246A-246D of leads 232, 240, 244 maydefine a three-lead full guard electrode configuration. The three-leadfull guard electrode combination utilizes electrodes on all three leadsimplanted within patient 12. In the example shown in FIG. 15B, electrodecombination 248 includes cathode electrode 234D on a middle lead 232,where the cathode electrode 234D is surrounded by two anode electrodes234C, 234E on the same lead 232 and anode electrodes 242B, 246B on leads240, 244 on either side of cathode electrode 234D. Anode electrodes234C, 234E, 242B, 246B of electrode combination 248 may define astimulation field that activates only the tissue around cathodeelectrode 235D while inhibiting the tissue on all sides of cathodeelectrode 235D.

FIG. 15C illustrates another example electrode combination 250 thatprocessor 110 of INS 26 may select during the second operating mode ofINS 26 in order to help focus the neurostimulation delivered to patient12. Electrode combination 250 is defined by electrodes 242A-242D and246A-246D of two leads 240, 244, respectively. In the example shown inFIG. 15C, electrodes 242A, 242C, 246B, 246D are anode electrodes andelectrodes 242B, 242D, 246A, 246D are cathode electrodes. Bysubstantially surrounding cathode electrodes 242B, 242D, 246A, 246C withanode electrodes 242A, 242C, 246B, 246D, electrode combination 250 mayshape a stimulation field that focuses stimulation to the area proximateleads 240, 244.

FIG. 15D illustrates another example electrode combination 256 that isdefined by selected electrodes 242A-242D, 232A-232H, 246A-246D of threeleads 240, 232, 244, respectively. In the example shown in FIG. 15D,electrodes 242A-242D, 246A-246D, 234A, 234C, 234E, 234G are anodeelectrodes and electrodes 234B, 234D, 234F, 234H are cathode electrodes.Anode electrodes 242A-242D, 246A-246D on leads 240, 244 adjacent to lead232, which includes cathode electrodes 234B, 234D, 234F, 234H, arepositioned to help limit the size of the stimulation field generated bythe delivery of electrical stimulation via electrode combination 256. Byplacing the cathode electrodes 234B, 234D, 234F, 234H along a centerlead 232, the stimulation field may be focused to the region of tissueproximate leads 232, 240, 244.

Anode electrodes 242A-242D, 246A-246D, 234B, 234D, 234F, 234H may act asguard band electrodes that help focus a stimulation field to the regionof tissue proximate leads 232, 240, 244. In some examples, anodeelectrodes 242A-242D may define a substantially continuous andcontiguous anode electrode, rather than a plurality of discreteelectrodes, as shown in FIG. 15D. Similarly, in some examples, anodeelectrodes 246A-246D may define a substantially continuous andcontiguous anode electrode, rather than a plurality of discreteelectrodes, as shown in FIG. 15D. Anode electrodes 242A-242D, 246A-246Don opposing sides of cathode electrodes 234B, 234D, 234F, 234H may serveas a guard band that reduce the projection of a stimulation field beyondleads 240, 246, which may help reduce the amount of the neurostimulationsignal that reaches the sense electrodes coupled to ICD 16. This mayhelp reduce the stimulation artifact on the sensing of cardiac signalsby ICD 16.

FIG. 15E illustrates another example electrode combination 260 that isdefined by electrodes 242A-242D, 232A-232H, 246A-246D on three leads240, 232, 244, respectively. In particular, cathode electrodes 234A-234Hare located on the middle (or central) lead 232, and anode electrodes242A-242D, 246-246D are positioned on leads 240, 244 on opposing sidesof center lead 232, which, in some examples, may be spatially centeredbetween leads 240, 244. Again, in some examples, anode electrodes242A-242D may define a substantially continuous and contiguous anodeelectrode and anode electrodes 246A-246D may define a substantiallycontinuous and contiguous anode electrode.

Anode electrodes 242A-242D, 246A-246D on opposing sides of cathodeelectrodes 234A-234H may serve as a guard band that reduce theprojection of a stimulation field beyond leads 240, 246, which may helpreduce the amount of the neurostimulation signal that reaches the senseelectrodes coupled to ICD 16. This may help reduce the stimulationartifact on the sensing of cardiac signals by ICD 16.

FIG. 15F illustrates another example electrode combination 262 that isdefined by electrodes positioned on four leads 240, 244, 264, 266 thatare coupled to INS 26, either directly or indirectly with a leadextension (e.g., a bifurcated lead extension). Electrodes 242A-242D oflead 240 may be anode electrodes and electrodes 246A-246D of lead 244may be cathode electrodes. Electrodes 268A-268D of lead 264 andelectrodes 270A-270D of lead 266 may be neutral, or inactive,electrodes. For example, electrodes 268A-268D may be electricallyconnected, e.g., shorted, to electrodes 270A-270D. Electrodes 268A-268D,270A-270D may limit the size (e.g., breadth) of the stimulation fieldgenerated by therapy delivery according to electrodes 242A-242D,246A-246D, e.g., by absorbing energy from the stimulation field.Minimizing the size of the stimulation field may help limit the extentto which ICD 16 senses the stimulation field, and, therefore, may helpminimize crosstalk between INS 26 and ICD 16.

In addition to or instead of modifying one or more operating parametersof INS 26, one or more operating parameters (e.g., sensing parameters)of ICD 16 may be modified in order to help prevent the inappropriatedelivery of stimulation by ICD 16 based on a neurostimulation signalartifact present in a signal sensed by that ICD 16. Modifying thesensing parameters of ICD 16 may help minimize the possibility that ICD16 mischaracterizes a neurostimulation signal as an electrophysiologicalcardiac signal. FIG. 16 is a flow diagram illustrating an exampletechnique that ICD 16 may implement in order to detect an arrhythmiawhile INS 26 is delivering electrical stimulation to a tissue site 40(FIG. 1) within patient 12.

Processor 90 of ICD 16 may determine whether INS 26 is deliveringstimulation to the nonmyocardial tissue site 40 (271). In some examples,INS 26 may transmit a signal to ICD 16 to notify ICD 16 that INS 26 isactively delivering electrical stimulation to patient 12, i.e., thedelivery of stimulation by INS 26 is not in a suspended state. Forexample, INS 26 may transmit a signal with predetermined characteristicsto ICD 16 via the respective telemetry modules 118, 98 prior to orsubstantially at the same time that INS 26 delivers a stimulation signalto patient 12 or at the beginning of a stimulation pulse train includingmore than one stimulation pulse.

As another example, ICD 16 may determine when INS 26 is deliveringstimulation based on a known stimulation schedule. As previouslyindicated, in some examples, ICD 16 and INS 26 have substantiallysynchronized clocks. Memory 92 (FIG. 6) of ICD 16 may store informationthat indicates when INS 26 is expected to be delivering stimulation topatient 12. For example, ICD 16 may store a stimulation schedule for INS26, where the stimulation schedule indicates the times of day at whichINS 26 is programmed to actively deliver stimulation to patient 12.

If INS 26 is delivering stimulation to patient 12, processor 90 of ICD16 may implement a first sense mode in order to monitor cardiac activityof patient 12 (272). The first sense mode may define a first sensingthreshold that is used by processor 90 (or sensing module 96, in someexamples) to detect a cardiac signal. ICD 16 may filter sensed signalswith the aid of the sensing threshold voltage in order to discriminatecardiac signals from noise, which may be attributable to many externalsources. Sensing module 96 may sense electrical signals via two or moreof the electrodes 50, 52, 54, 56, 58, 60, 68, 72, 74, 76 connected tosensing module 96. Processor 90 may only identify sensed signals thathave a voltage amplitude greater than the sensing threshold value aselectrical cardiac activity. For example, sensing module 96 may onlytransmit EGM signals above the sensing threshold value to processor 90for timing analysis. As previously indicated, the timing analysis mayinclude an analysis of the sensed EGM signal for R-R intervals, P-Pintervals, and so forth.

If INS 26 is not delivering stimulation to patient 12, e.g., because thedelivery of stimulation by INS 26 is currently suspended, processor 90may implement a second sense mode in order to sense cardiac signals(273). The second sense mode may define a second sensing threshold thatis used by processor 90 (or sensing module 96, in some examples) todetect a cardiac signal. In some examples, the second sensing thresholdmay be lower than the first sensing threshold defined by the first sensemode. In this way, the second sense mode may be more sensitive toelectrical cardiac signals than the first sense mode.

In some examples, the first and second sense modes may also definedifferent amplifier gains used by the sensing amplifiers of sensingmodule 96 to sense electrical cardiac signals. The first sense mode mayhave a lower amplifier gain than the second sense mode, which may resultin less sensitivity to cardiac signals.

While the first sense mode of ICD 16 may be less sensitive to electricalcardiac signals of patient 12, ICD 16 may monitor other physiologicalparameters of patient in order to detect an arrhythmia, thereby at leastpartially compensating for the decreased sensitivity to electricalcardiac signals. That is, in the first sense mode, in addition tosensing electrophysiological cardiac signals (e.g., EGM or ECG signals)of patient 12, processor 90 may detect an arrhythmia based on othernon-electrophysiological parameters that are indicative of cardiacactivity of patient in order to detect an arrhythmia. In contrast, inthe second sense mode, sensing module 96 may not detect an arrhythmiabased on non-electrophysiological parameters of patient 12 or may detectan arrhythmia based on fewer non-electrophysiological parameters ofpatient 12 compared to the second sense mode.

Examples of non-electrophysiological parameters of patient 12 that maybe indicative of an arrhythmia include, but are not limited to,cardiovascular pressure, tissue perfusion, blood oxygen saturationlevels, heart sound signals, respiratory rate, thoracic impedance,cardiac mechanical activity (e.g., muscle movement monitored via anaccelerometer), body temperature (e.g., metabolic rate may change withdecreased cardiac function, which may affect body temperature), acousticsignals indicative of cardiac mechanical activity or other blood flowinformation. Sensing a greater number of non-electrophysiologicalparameters of patient 12 in the first sense mode may help preventunderdetecting an arrhythmia of patient 12 despite the less sensitivesensing threshold utilized to sense cardiac signals.

Cardiovascular pressure may include intracardiac pressure (i.e.,pressure within a chamber of heart 14) or extravascular pressure sensedoutside of the patient's vasculature. One or more characteristics ofsensed cardiovascular pressure in either the time domain or frequencydomain may indicate whether a detected arrhythmia is a true arrhythmia.Cardiovascular pressure may vary based on the mechanical contraction andrelaxation of heart 14. Thus, changes in cardiovascular pressure mayindicate whether heart 14 is mechanically contracting and relaxing in anormal manner and, therefore, may indicate the presence of anarrhythmia. For example, an arrhythmia may be detected if the timedomain cardiovascular pressure data indicates that the pressure withinright ventricle 32 (FIG. 3) of heart 14 has decreased by at least aparticular amount or decreased below a threshold amount. As anotherexamples, processor 90 of ICD 16 may detect an arrhythmia using thefirst sense mode of ICD 16 if the pressure within right ventricle 32(FIG. 3) or another chamber is less than its expected physiologic range.

Intracardiac pressure may be monitored with the aid of a pressure sensorcoupled to at least one of leads 18, 20, 22. Extravascular pressure maybe monitored with the aid of a pressure sensor located outside of heart14. The pressure sensor may be mechanically coupled to or physicallyseparate from ICD 16 and INS 26. If physically separate from ICD 16 andINS 26, the pressure sensor may transmit a signal indicative of pressureto ICD 16 and INS 26 via a wired or wireless connection. The sensedpressure may be, for example, a systolic pressure, diastolic pressure, apulse pressure, a maximum and minimum derivative of sensed pressure(s),or any combination thereof.

Tissue perfusion and blood oxygen saturation levels of patient 12 mayalso vary based on the mechanical contraction and relaxation of heart14. Thus, changes in tissue perfusion or blood oxygen saturation levelsor a decreasing trend in blood oxygen saturation or tissue perfusion mayindicate that heart 14 is not mechanically contracting and relaxing in anormal manner and, therefore, may indicate the presence of anarrhythmia. Tissue perfusion and blood oxygen saturation levels ofpatient 12 may be with the aid of an optical sensor, which may or maynot be mechanically coupled to ICD 16 or INS 26.

As described in U.S. Patent Application Publication No. 2007/0239215 toBhunia et al., entitled, “METHOD AND APPARATUS FOR USING AN OPTICALHEMODYNAMIC SENSOR TO IDENTIFY AN UNSTABLE ARRHYTHMIA,” which was filedon Mar. 31, 2006 and is incorporated herein by reference in itsentirety, an optical perfusion sensor may include a red light emittingdiode (LED) and an infrared (IR) LED as light sources, and a detector.An increase in a red optical signal sensed by the detector, which mayindicate the amount of red light from the red LED that was reflected byblood in the tissue proximate to the optical perfusion sensor, and adecrease in an IR signal sensed by the detector, which may indicate theamount of IR light form the IR LED that was reflected by blood in thetissue in blood-perfused tissue, may indicate the occurrence of acardiac arrhythmia. According to U.S. patent application Ser. No.11/394,477 to Bhunia et al., electrical signals generated by thedetector of the optical perfusion sensor may experience a significantchange in value following a hemodynamically unstable ventricularfibrillation. This change may be detected by sensing module 96 or aseparate optical sensor in the second sense mode of sensing module 96 inorder to detect an arrhythmia.

Another non-electrophysiological parameter of patient 12 that processor90 may use to detect an arrhythmia in the first sense mode includesheart sound signals or acoustic signals indicative of mechanicalcontractions of heart 14. Heart sounds or other acoustic signals may besensed with a sensor, which may or may not be coupled to ICD 16 or INS26, such as an accelerometer or acoustic transducer. The heart sounds oracoustic vibrations may be generated as the heart valves open and closeduring a cardiac cycle or by turbulent flow during the fill phases indiastole. Changes in the heart sounds or acoustic vibrations, such asthe lack of heart sounds or acoustic vibrations or a decrease in thefrequency of the heart sounds or acoustic vibrations may indicate thepresence of an arrhythmia.

In some examples, in either or both the first and second sense modes,sensing module 96 or processor 90 may filter out the neurostimulationsignals from sensed electrical signals. For example, sensing module 96may implement a front-end filter to filter out the neurostimulationsignals delivered by INS 26 or processor 90 may implement digital signalprocessing to filter out the neurostimulation signals. Because thesource of the artifact from the electrical signals generated anddelivered by INS 26 is known, and the characteristics of the electricalsignals are known, it may be relatively easy for processor 90 to filterout the electrical signals generated and delivered by INS 26. Forexample, sensing module 96 may filter sensed signals on the basis offrequency content and eliminate frequency components of a sensed signalthat falls outside of the range. The neurostimulation signal deliveredby INS 26 may have a known signature, in terms of the signal frequency,duty cycle, signal envelope, and so forth.

ICD 16 may store the known signature in memory 92 or INS 26 mayperiodically provide the neurostimulation signal information to ICD 16.For example, INS 26 may periodically transmit the therapy programdefining the stimulation parameter values with which INS 26 generateselectrical stimulation signals. In some examples, ICD 16 may sensecardiac signals while INS 26 is delivering stimulation signals topatient 12, and processor 90 may determine the characteristics (e.g.,patterns, amplitude, frequency, and the like) of the neurostimulationsignal artifact present in the sensed signal. This may be done, forexample, after ICD 16 and INS 26 are implanted within patient 12, e.g.,in the same session. In this way, processor 90 of ICD 16 may learn thecharacteristics of the neurostimulation signal artifact that may bepresent in a sensed signal.

Processor 90 of ICD 16 may use these known characteristics of theneurostimulation signal to filter the signal out of the electricalsignals sensed by sensing module 96. In some examples, processor 90 orsensing module 96 may include a notch filter to filter theneurostimulation signals generated by INS 26. The notch filter maycomprise a band-stop filter (or a band rejection filter) that attenuatesfrequencies in a specific frequency range. The frequency range of thenotch filter may be selected based on the known frequency range of theneurostimulation signals generated and delivered by INS 26. The notchfilter may be dynamically adjustable based on, for example, the therapyprogram with which INS 26 generates the electrical stimulation signals.

In some examples, sensing module 96 of ICD 16 may apply differentfilters to sensed electrical signals in the first and second sensemodes. In addition, in some examples, processor 90 of ICD 16 may applydifferent arrhythmia detection algorithms based on whether the first orsecond sense modes are applied by ICD 16. The arrhythmia detectionalgorithms may define the rules with which processor 90 identifies apotential arrhythmia. For example, the arrhythmia detection algorithmsmay define the number of arrhythmia events that define an arrhythmiaepisode, or the R-R interval duration that defines an arrhythmia event.

Modifying the type of arrhythmia detection algorithms based on whetherINS 26 is delivering stimulation to patient 12 may help compensate forthe decrease in sensitivity to electrical cardiac signals in the firstsense mode of ICD 16 compared to the second sense mode. For example,when ICD 16 is applying the first sense mode to sense electrical cardiacsignals, processor 90 of ICD 16 may determine that an arrhythmia episodeis observed when a fewer number of R-R intervals having a duration lessthan a stored threshold are detected compared to arrhythmia detectionalgorithm implemented during the second sense mode. In this way,processor 90 may compensate for the decrease in sensitivity toelectrical cardiac signals by increasing the sensitivity to arrhythmiadetection.

In some examples, the segment of an electrical cardiac signal that isobserved to detect the arrhythmia may differ based on whether ICD 16 isapplying the first or second sense modes. For example, in the firstsense mode, processor 90 of ICD 16 may detect arrhythmia events based ona duration of an S-T segment of a sensed EGM, and in the second sensemode, processor 90 may detect arrhythmia events based on a differentsegment of a sensed EGM (e.g., the R-R segment or P-P segment).

FIG. 17A provides a conceptual illustration of an ECG signal 274 sensedby a sensing device via subcutaneous electrodes on left and right sidesof a human subject. ECG signal is an example of an electrical signalthat is sensed prior to the application of a filter by a processor(e.g., processor 90 of ICD 16). FIG. 17B provides a conceptualillustration of filtered ECG signal 276 after a processor applies afilter to sensed ECG signal 274. An artifact from delivery ofneurostimulation is present in ECG signal 274. As FIG. 17B demonstrates,sensed ECG signal 274 comprising the neurostimulation signal artifactexhibits a relatively fast heart rhythm, e.g., about 260 beats perminute. Signal processing ECG signal 274, e.g., by applying a filter toECG signal 274, may help remove the relatively high frequencyneurostimulation signal artifact from sensed ECG signal 274. As FIG. 17Billustrates, the processed ECG signal 276 exhibits a relatively slowerheart rhythm, such as about 92 beats per minute.

The processed ECG signal may be a more accurate and preciserepresentation of true cardiac signals of the human subject. Forexample, while the heart rhythms indicated by signal 274 may indicate aventricular tachycardia events, the processed signal 276 indicates aslower heart rhythm, which may not be associated with a ventriculartachycardia events. Accordingly, it may be useful for processor 90 toapply one or more filters or implement other signal processingtechniques to a sensed signal in order to minimize the possibility ofdelivering inappropriate therapy to patient 12.

FIG. 18A provides a conceptual illustration of an ECG signal 278 sensedby a sensing device via subcutaneous electrodes on left and right sidesof a human subject. An ischemia-inducted ventricular tachycardia wasinduced in the human subject. FIG. 18B provides a conceptualillustration of filtered ECG signal 280 after a processor applies afilter to sensed ECG signal 278. An artifact from delivery ofneurostimulation is present in ECG signal 278. As FIG. 18A demonstrates,sensed ECG signal 278 comprising the neurostimulation signal artifactexhibits a relatively fast heart rhythm, e.g., about 470 beats perminute. Signal processing ECG signal 278, e.g., by applying a filter toECG signal 278, may help remove the relatively high frequencyneurostimulation signal artifact from sensed ECG signal 278. As FIG. 18Billustrates, the processed ECG signal 280 exhibits a relatively slowerheart rhythm, such as about 280 beats per minute. FIGS. 18A and 18Bfurther demonstrate that a processed ECG signal 280 may be a moreaccurate and precise representation of true cardiac signals of the humansubject. As FIGS. 18A and 18B demonstrate, at least partially filteringthe neurostimulation signal artifact from a sensed electrical signal maybe useful for determining a true heart rate, such as a true ventriculartachycardia rate.

FIG. 19A is a flow diagram illustrating another example technique thatprocessor 90 of ICD 16 may implement in order to change a cardiac signalsense mode based on whether INS 26 is actively delivering electricalstimulation. As with the technique shown in FIG. 16, processor 90 maydetermine whether INS 26 is delivering stimulation to patient 12 (271).If INS 26 is currently delivering stimulation to patient 12, processor90 may control sensing module 96 to sense cardiac signals via a firstsense mode (272). On the other hand, if INS 26 is not deliveringstimulation to patient 12, e.g., because the delivery of stimulation byINS 26 is currently suspended, processor 90 may implement a second sensemode in order to sense cardiac signals (273), where the second sensemode comprises at least one different sensing parameter than the firstsense mode. In addition, if processor 90 (or sensing module 96 under thecontrol of processor 90) detects an arrhythmia via the first sense mode(284), processor 90 may control INS 26 to suspend the delivery oftherapy to patient 12 (285) and processor 90 may control sensing module96 to sense according to the second sense mode (273).

If processor 90 detects an arrhythmia while sensing cardiac activity viathe second sensing mode (286), processor 90 may control stimulationgenerator 94 (FIG. 6) to deliver the appropriate stimulation therapy toheart 14 (288), which may be, for example, any one or more of pacing,cardioversion or defibrillation pulses. As shown in FIG. 19A, the secondsense mode of ICD 16 may be used to confirm the detection of anarrhythmia detected via the first sense mode. The second sense mode ofICD 16 may be more specific to appropriately detecting electricalcardiac signals than the first sense mode, e.g., may be more likely todetect a cardiac arrhythmia based on the electrical cardiac signalscompared to the first sense mode. This may be attributable to, forexample, the lower cardiac signal sensing threshold defined by thesecond sense mode and/or the higher amplifier gain used to sense thesignals. By decreasing the sensing threshold or increasing the amplifiergain, the sensitivity of ICD 16 to heart signals may increase becausesensing module 96 may characterize more electrical signals as cardiacsignals, and, therefore decrease the possibility of undersensing cardiacsignals.

As previously indicated, although the first sense mode is less sensitiveto cardiac signals, the first sense mode detects an arrhythmia based onother physiological parameters of patient. Detecting a potentialarrhythmia based on physiological parameters in addition to electricalcardiac signals may compensate for the decrease in sensitivity tocardiac signals.

As shown in FIG. 19A, the second sense mode may be a default sense modewhen INS 26 is not actively delivering stimulation therapy to patientbecause crosstalk between ICD 16 and INS 26 may be negligible. Thus, thepossibility that sensing module 96 may oversense cardiac signals in thesecond sense mode is reduced when INS 26 is not actively deliveringstimulation therapy to patient 12.

In some examples, the first and second sense modes may comprisedifferent sense vectors. A sense vector may be defined by the subset ofelectrodes 50, 52, 54, 56, 58, 60, 70, 72, 74, and 76 electricallycoupled to ICD 16 that are used by sensing module 96 to sense electricalcardiac signals. A sensing vector may be modified by switching theelectrodes with which sensing module 96 senses intracardiac electricalsignals. ICD 16 may sense electrical cardiac signals via one or moreexternal electrodes. In some examples, ICD 16 may sense electricalcardiac signals via external electrodes in the first sense mode andsense electrical cardiac signals via implanted electrodes in the secondsense mode.

As another example of how a sensing vector may be modified by selectingdifferent electrode, if sensing module 96 senses an intracardiacelectrical signal via electrodes 50, 52 of lead 18, which are positionedin right ventricle 32 (FIG. 3), and processor 90 detects an arrhythmia(284) based on the sensed signals, processor 90 may control sensingmodule 96 to switch sense modes, and, therefore, switch sensing vectorsand sense intracardiac electrical signals via electrodes 54, 56 of lead20, which is positioned in left ventricle 36 (FIG. 3).

In some examples, sensing module 96 of ICD 16 may sense electricalcardiac signals within left ventricle 36 (FIG. 3) and outside of rightventricle 32 (FIG. 3) of heart 14 in the first sense mode. That is, inthe first sense mode, sensing module 96 may not sense electrical cardiacsignals via electrodes 50, 52, 72 (FIG. 3) positioned within rightventricle 32. In addition, in some examples, sensing module 96 of ICD 16may sense electrical cardiac signals within right ventricle 32 andoutside of left ventricle 36 of heart 14 in the first sense mode. Thatis, in the first sense mode, sensing module 96 may not sense electricalcardiac signals via electrodes 54, 56, 74 (FIG. 3) positioned withinleft ventricle 36.

As another example of how ICD 16 may switch sense vectors with whichelectrical cardiac signals of heart 14 of patient 12 are sensed, in thefirst sense mode, ICD 16 may sense electrical cardiac signals via twoelectrodes of one of leads 18, 20, 22 (FIG. 3), and in the second sensemode, ICD 16 may sense electrical cardiac signals via at least oneelectrode carried by a lead 18, 20 and/or 22 and housing electrode 68(FIG. 3). In this way, in the second sense mode, ICD 16 may senseelectrical cardiac signals across a greater span of heart 14 than in thefirst sense mode.

In some examples, in at least the first sense mode, ICD 16 may senseelectrical cardiac signals via each of a plurality of sense vectors. IfICD 16 senses electrical cardiac signals via each of a plurality ofsense vectors in the second sense modes, the sense vectors defined bythe second sense mode may be different than the sense vectors defined bythe first sense mode. Crosstalk from therapy delivery by INS 26 may havedifferent strengths, depending on the vector with which ICD 16 senseselectrical signals. Thus, sensing electrical cardiac signals with aplurality of sense vectors may help increase the possibility that ICD 16senses a true electrical cardiac signal or at least an electricalcardiac signal that that does not have a large signal artifact from INScrosstalk.

In addition, if ICD 16 senses electrical cardiac signals via each of aplurality of sense vectors, ICD 16 may determine cardiac function ofpatient 12 based on a weighted sum of the electrical cardiac signals orat least based on a correlation of the electrical cardiac signals sensedvia two or more sense vectors. In one example of weighing the electricalsignals sensed by each of a plurality of sensing vectors, processor 90may individually gain and sum the signals and detect cardiac episodes orevents (e.g., a tachyarrhythmia) based on the summed signal. In anotherexample, processor 90 may sum the absolute value of each sensed signal.In general, processor 90 sums the different sensed signals in order tocombine the sensing information and attempt to filter out crosstalknoise, which may only be affecting only one or two of the sensingvectors.

In some examples, the electrical signals sensed via each of the sensevectors are each used to determine the timing of the R-waves or othersignal characteristics, e.g., to detect an arrhythmia. Processor 90 ofICD 16 may determine whether the R-waves sensed via different sensevectors indicate that an arrhythmia is detected. If, for example, athreshold number (e.g., two or more) of the electrical signals sensedvia different sense vectors indicate different R-R intervals, processor90 may determine that the sensed electrical cardiac signals are not trueelectrical cardiac signals, but are at least partially attributable todelivery of electrical stimulation by INS 26.

If processor 90 detects a potential arrhythmia based on intracardiacelectrical signals sensed via a first sensing vector defined by thefirst sense mode (284), processor 90 may determine whether an arrhythmiais detected based on the intracardiac electrical signals sensed via asecond sensing vector defined by the second sense mode (273, 284). Thefirst and second sensing vectors may be defined by respective subsets ofelectrodes 50, 52, 54, 56, 58, 60, 70, 72, 74, and 76, where the firstand second sensing vectors may include at least one different electrode.If processor 90 detects the potential arrhythmia based on the signalssensed via the new sensing vector, processor 90 may confirm the presenceof the arrhythmia and, therefore, determine whether the detectedarrhythmia was based on true cardiac signals, or at least not based onelectrical stimulation signals from INS 26.

In some cases, switching the sensing vector may help decrease thecrosstalk that ICD 16 senses by, for example, changing the relativevector between the stimulation electrodes 124 connected to INS 26 andthe sensing vector used by ICD 16 to sense cardiac signals. Thus, insome cases, the crosstalk sensed by the new sensing vector may changecharacteristics compared to the initial sensing vector, and, as aresult, processor 90 may not mischaracterize the artifact generated bythe delivery of electrical stimulation by INS 26 as true cardiacsignals.

If the potential arrhythmia is not detected (286) when ICD 16 is sensingin the second sense mode, processor 90 may determine that the potentialarrhythmia detected via the cardiac signals sensed via the first sensemode (284) was a false detection based on noise from INS 26, rather thantrue cardiac signals. Thus, processor 90 may not provide any therapy topatient 12, and processor 90 of ICD 16 may determine whether INS 26 isdelivering electrical stimulation to patient 12 (271) and controlsensing module 96 to sense electrical cardiac signals of patient 12 viathe first sense mode (272) if INS 26 is delivering stimulation topatient 12 and control sensing module 96 to sense electrical cardiacsignals of patient 12 via the second sense mode if INS 26 is notdelivering stimulation to patient 12 (273).

FIG. 19B is a flow diagram illustrating another example technique thatprocessor 90 of ICD 16 may implement in order to change a cardiac signalsense mode based on whether INS 26 is actively delivering electricalstimulation. The technique shown in FIG. 19B is similar to that shown inFIG. 19A. However, in the example shown in FIG. 19B, if processor 90 (orsensing module 96 under the control of processor 90) detects anarrhythmia via the first sense mode (284), in the example shown in FIG.19B, processor 90 may control stimulation generator 94 (FIG. 6) todeliver the appropriate stimulation therapy to heart 14 (289). Incontrast, in the example shown in FIG. 19A, processor 90 controlled INS26 to suspend or otherwise adjust the delivery of stimulation to patient12 (285) and then determined whether the potential arrhythmia was alsodetected when the patient's condition was sensed via the second sensemode of ICD 16 (273, 286).

In some cases, it may be desirable to evaluate the extent of thecrosstalk between INS 26 and ICD 16. For example, it may be desirable toevaluate the strength of the electrical stimulation signal generated byINS 26 and sensed by ICD 16, i.e., evaluate one or more characteristicsof an artifact present in a signal sensed by ICD 16 when INS 26 isdelivering stimulation to patient 12. The artifact may be referred to asa neurostimulation artifact, although the artifact may also beattributable to the delivery of stimulation other than neurostimulationby INS 26. A clinician or patient 12 may evaluate the crosstalk betweenINS 26 and ICD 16 in order to determine if the crosstalk is excessive atvarious times, such as after implantation of ICD 16 and INS 26 inpatient 12, after programming the electrical stimulation parameters orsensing parameters of either ICD 16 or INS 26 or periodically throughoutthe use of therapy system 10.

Crosstalk may be excessive if it hinders the intended operation of ICD16, such as the sensing of true cardiac signals by ICD 16. As previouslydescribed, in some examples, ICD 16 may sense the electrical stimulationsignal generated and delivered by INS 26 and mischaracterize theelectrical stimulation signal as a cardiac signal. Thismischaracterization of the electrical stimulation signal as a cardiacsignal may result in a detection of a cardiac arrhythmia, which mayresult in the inappropriate delivery of a defibrillation shock or otherelectrical stimulation to heart 14. In this way, the crosstalk betweenINS 26 and ICD 16 may affect the intended operation of ICD 16.

In some examples, the crosstalk between INS 26 and ICD 16 may beexcessive if a characteristic of a signal sensed by the ICD 16 whileelectrical stimulation is being delivered by INS 26 differs from acharacteristic of a baseline signal by a threshold value. As describedin further detail below, the characteristic of the electrical signalsmay be an amplitude value or a power level (or energy level) in one ormore frequency bands. For example, the characteristic of the electricalsignals may be an absolute amplitude value or a root mean squareamplitude value. In addition, the amplitude value may comprise a mean ormedian amplitude value over a period of time or a maximum amplitude oran amplitude in a particular percentile of the maximum (e.g., anamplitude value that represents 95% of the maximum amplitude value). Insome examples, as described in further detail below, the threshold valuemay be a percentage of a sensing threshold with which ICD 16 senseselectrical cardiac signals.

If the crosstalk between INS 26 and ICD 16 is determined to beexcessive, a clinician or a device (e.g., INS 26, ICD 16 or programmer24) may attempt to reduce the extent of the crosstalk. For example, ICD16 or INS 26 may modify one or more stimulation parameter values of INS26, as described with respect to FIGS. 9-12B and/or modify one or moresensing parameter values of ICD 16, as described with respect to FIGS.16, 19A, and 19B.

In some examples, an external device, such as medical device programmer24 (FIG. 1) may be used to evaluate the extent of crosstalk between INS26 and ICD 16. While programmer 24 is primarily referred to throughoutthe description of FIG. 20, in other examples, another device may beused to measure the amount of crosstalk between INS 26 and ICD 16. Thedevice may be an external device, such as multifunction computing deviceor may be a device dedicated to measuring the amount of crosstalkbetween INS 26 and ICD 16, or one of the implanted medical devices 16,26. In addition, in some examples, ICD 16, INS 26 or another implanteddevice may measure the amount of crosstalk between ICD 16 and INS 26.The implanted device may store the information indicative of the amountof crosstalk between ICD 16 and INS 26 or may transmit information to anexternal device, such as programmer 24.

FIG. 20 is a flow diagram illustrating an example technique forevaluating crosstalk between ICD 16 and INS 26. The technique shown inFIG. 20 may be implemented in order to determine a status of theelectrical noise sensed by ICD 16 due to the delivery of stimulation byINS 26. The status determination may be used to, for example, modify anstimulation parameter of INS 26 or a sensing parameter of ICD 16, e.g.,in accordance with the techniques described above with respect to FIGS.9-12, 16, 19A, and 19B.

As shown in FIG. 20, processor 130 of programmer 24 (FIG. 8) mayevaluate one or more characteristics of a signal sensed by ICD 16 whenthe neurostimulation artifact is present under the direction of aclinician or automatically, e.g., based on a schedule determined by aclinician. The schedule may define an evaluation frequency withprocessor 130 evaluates the neurostimulation signal artifact sensed byICD 16. For example, the artifact evaluation frequency may be in a rangeof about one to about ten times per minute, once per hour, or once perday, although other frequency ranges are contemplated.

In order to measure the magnitude of the neurostimulation artifact (or“crosstalk”) present in the electrical cardiac signal sensed by ICD 16,processor 130 of programmer 24 may instruct processor 110 of INS 26 tosuspend or otherwise adjust the delivery of neurostimulation (290). Forexample, processor 130 of programmer 24 may transmit a control signal toprocessor 110 via the respective telemetry modules 136 (FIG. 8), 118(FIG. 7). The control signal may not only indicate whether INS 26 shouldsuspend or otherwise adjust the delivery of neurostimulation to patient12, but, in some examples, may indicate how long INS 26 should suspendneurostimulation or deliver therapy according to the adjust parameters.In other examples, memory 112 (FIG. 7) of INS 26 may store instructionsfor suspending or otherwise adjusting neurostimulation when processor110 of INS 26 receives the control signal from processor 130 ofprogrammer 24. As another example, INS 26 may suspend or otherwiseadjust delivery of stimulation without intervention from programmer 24,e.g., according to schedule stored by memory 112.

During the time in which neurostimulation is suspended or adjusted,processor 130 of programmer 24 may receive an electrical signal sensedby ICD 16 from ICD 16 (292). This electrical signal may represent abaseline artifact level present in the cardiac signal sensed by ICD 16.Artifacts from sources other than the neurostimulation signals deliveredby INS 26 may be present in the signal sensed by ICD 16, such as fromelectromagnetic interference from electronics or electrical outlets inthe patient's surroundings. The baseline electrical signal may indicatethese other artifacts present in the signal sensed by ICD 16.

In some examples, processor 130 of programmer 24 may instruct processor90 of ICD 16 to sense a baseline electrical signal via a selectedsensing channel of sensing module 96 (FIG. 6) of ICD 16. As describedwith respect to FIG. 6, in some examples, sensing module 96 may includea plurality of sensing channels, which may each include an amplifier.For example, sensing module 96 may include a sensing channel includingan R-wave amplifier to sense R-waves within right ventricle 32 of heart14 (FIG. 3), a sensing channel including an R-wave amplifier to senseR-waves within left ventricle 36 of heart 14 (FIG. 3), a sensing channelincluding a P-wave amplifier to sense P-waves within right atrium 30 ofheart 14 (FIG. 3), and/or a sensing channel including a wide bandamplifier in order to generate an EGM representing the electricalactivity of heart 14. Processor 90 of ICD 16 may transmit the electricalsignal sensed on the selected sensing channel of sensing module 96 toprocessor 130 of programmer 24 via the respective telemetry modules 98(FIG. 6), 136 (FIG. 8).

After processor 130 of programmer 24 receives the baseline electricalsignal from ICD 16 (292), processor 130 may control processor 110 of INS26 to activate the delivery of electrical stimulation (294). Forexample, processor 130 may generate a control signal that is transmittedto processor 110 of INS 26 via the respective telemetry modules 136(FIG. 8), 118 (FIG. 7). Upon receiving the control signal, processor 110of INS 26 may control stimulation generator 114 to begin generating anddelivering neurostimulation therapy, e.g., in accordance with a firstoperating mode of INS 26. As described with respect to FIG. 12A, a firstoperating mode may be defined by a therapy program that defines one ormore stimulation parameter values for the electrical stimulation signalsgenerated and delivered by INS 26. In other examples, processor 110 ofINS 26 may begin generating and delivering neurostimulation therapybased on a predetermined schedule that indicates the times at whichprocessor 110 should suspend the delivery of neurostimulation andinitiate the delivery of stimulation.

After INS 26 commences the delivery of neurostimulation to patient 12,processor 130 of programmer 24 may receive an electrical signal sensedby the selected channel of sensing module 96 of ICD 16 (FIG. 6) (296).This electrical signal that is sensed on the selected sensing channelduring the delivery of neurostimulation by INS 26 may be referred to asa “second electrical signal” to distinguish it from the baselineelectrical signal. Processor 130 of programmer 24 may receive thebaseline electrical signal and the second electrical signal, forexample, by periodically interrogating ICD 16. In other examples, ICD 16may periodically transmit the baseline electrical signal and secondelectrical signal to processor 130 of programmer 24 without beinginterrogated by programmer 24.

Processor 130 of programmer 24 may determine the neurostimulation signalartifact on the selected sensing channel of ICD 16 based on the baselineelectrical signal and the second electrical signal that was sensed whileINS 26 was actively delivering neurostimulation to patient 12 (298). Insome examples, processor 130 of programmer 24 may determine theneurostimulation signal artifact that is present on more than onesensing channel of sensing module 96 of ICD 16. In addition, in someexamples, processor 90 of ICD 16 may sense the neurostimulation signalartifact present in the signal sensed via one or more selected sensingchannels during a quiet segment of the cardiac cycle. The quiet segmentof a cardiac cycle may be when the intrinsic electrical signal of heart14 is least active, such as during the S-T segment of a sinus rhythm ofheart 14.

In some examples, processor 130 of programmer 24 may determine theneurostimulation signal artifact on the selected sensing channel bydetermining a difference between one or more signal characteristics ofthe baseline electrical signal and the second electrical signal. In someexamples, the signal characteristic may comprise a current or a voltageamplitude of the signal waveforms. For example, processor 130 ofprogrammer 24 may determine a difference in the amplitude of thebaseline electrical signal and a sensing threshold of sensing module 96(FIG. 6) of ICD 16. This value may be referred to as the “first value”for ease of description. The amplitude may be a mean or median amplitude(e.g., a peak-to-peak amplitude), a highest amplitude (e.g., a greatestpeak-to-peak amplitude), a root means square (RMS) amplitude, anamplitude that is equal to a certain percentage (e.g., about 95%) of theamplitude, and the like. A sensing threshold may indicate a thresholdamplitude value above which processor 90 of ICD 16 characterizes asensed electrical signal as an electrical cardiac signal.

Processor 130 may also determine a second value indicative of thedifference in the amplitude of the second electrical signal and asensing threshold of sensing module 96. The amplitude may be a mean ormedian amplitude, a highest amplitude, a RMS amplitude, an amplitudethat is equal to a certain percentage (e.g., about 95%) of the highestamplitude, and the like. In order to determine the neurostimulationsignal artifact on the selected sensing channel, processor 130 ofprogrammer 24 may determine a difference between the first and secondvalues. If the difference is greater than or equal to a stored thresholdvalue, which may be based on the sensing threshold amplitude of ICD 16,processor 130 may determine that the crosstalk between ICD 16 and INS 26due to the delivery of neurostimulation by INS 26 is unacceptable. Onthe other hand, if the difference between the first and second values isless than the stored threshold value, processor 130 may determine thatthe crosstalk between ICD 16 and INS 26 due to the delivery ofneurostimulation by INS 26 is within acceptable ranges. In this way,processor 130 may evaluate the extent of the crosstalk between ICD 16and INS 26 due to the delivery of neurostimulation by INS 26. Thethreshold value may be, for example, selected by a clinician and storedby programmer 24, ICD 16, INS 26 or another device.

As another example, the signal characteristic may comprise a power levelwithin a particular frequency band of an electrical signal. Processor130 may determine the neurostimulation signal artifact by determining afirst value indicative of the difference in energy levels in theselected frequency band of the baseline electrical signal and a storedenergy level, and a second value indicative of the difference in energylevels in the selected frequency band of the second electrical signaland the stored energy level. The difference between the first and secondvalues may be indicative of noise on a sensing channel of ICD 16 due tothe delivery of neurostimulation by INS 26.

Processor 130 of programmer 24 may display data indicative of the extentof crosstalk between ICD 16 and INS 26 on a display of user interface134 (FIG. 8) (299). For example, the data may include a graphicaldisplay of the waveform of the baseline electrical signal or a waveformof the second electrical signal. An example of a graphical display ofdifferent types of waveforms indicative of the crosstalk between ICD 16and INS 26 is shown in FIG. 21, which is described below.

As patient 12 changes posture and/or activity level, the one or moreleads 28, 29 (FIGS. 1 and 2) connected to INS 26 may move within patient12. For example, in the example shown in FIG. 2, as patient 12 changesposture, leads 28, 29 may move relative to ICD 16 as spinal cord 44moves. The amount of neurostimulation artifact that ICD 16 senses maychange as a function of the position of leads 28, 29 within patient 12.For example, in some patient postures, at least one of the leads 28, 29may be closer to the sense electrodes coupled to ICD 16, and, as aresult, ICD 16 may sense a stronger neurostimulation signal. That is, asleads 28, 29 move closer to ICD 16, the extent of crosstalk between INS26 and ICD 16 may increase. Similarly, for increased levels of patientactivity, leads 28, 29 may undergo more movement within patient 12,which may also result in at least one of the leads 28, 29 moving closerto heart 14.

In some examples, in order to better evaluate the neurostimulationartifact present in a signal sensed by ICD 16 when INS 26 is activelydelivering stimulation, processor 130 of programmer 24 may evaluate theneurostimulation artifact while patient 12 is in different posturesand/or activity levels. This may help processor 130 and/or the clinicianevaluate the spectrum of crosstalk that may be present between ICD 16and INS 26. In some examples, processor 130 may present a display onuser interface 134 (FIG. 8) of programmer 24 that prompts patient 12 toundertake different postures or activities. The different patientpostures may include, for example, standing, sitting, a prone position,bending forward while standing or bending backward at the waist whilestanding, and the like. Processor 130 may then evaluate the amount ofcrosstalk between INS 26 and ICD 16 while patient 12 is in each of thedifferent postures or activities, e.g., using the technique shown inFIG. 20. For example, while patient 12 is in each of the differentpostures or activities, processor 130 of programmer 24 may receive andrecord both a baseline and a second electrical signal sensed by ICD 16.

In other examples, processor 90 of ICD 16, rather than processor 130 ofprogrammer 24, may determine the neurostimulation signal artifact on theselected sensing channel of ICD 16 based on the baseline electricalsignal and the second electrical signal that was sensed while INS 26 wasactively delivering neurostimulation to patient 12. In this way, ICD 16may provide real-time detection of crosstalk and switch sensing modes ata useful time, e.g., before inappropriately delivering a shock topatient 12, or communicate to the INS 26 to adjust therapy delivery(e.g., adjust a stimulation parameter value or suspendneurostimulation).

FIG. 21 is a flow diagram illustrating an example technique that may beused to evaluate the extent of the crosstalk between INS 26 and ICD 16and minimize the crosstalk if the crosstalk exceeds a threshold level.Processor 130 of programmer 24 may measure the crosstalk (300), e.g.,using the technique described with respect to FIG. 20. Processor 130 maydetermine whether the extent of crosstalk exceeds a threshold level(302). In some examples, processor 130 may determine whether the extentof crosstalk exceeds the threshold level by determining whether thevalues of one or more signal characteristics (e.g., a voltage amplitude)of the second electrical signal differs from the respective signalcharacteristic values of the baseline electrical signal. The thresholdlevel may indicate a percentage change or an absolute value change thein one or more signal characteristics. As discussed with respect to FIG.20, the baseline electrical signal may represent the amount of artifactpresent on a selected sensing channel of ICD 16 when the delivery ofneurostimulation by INS 26 is suspended and the second electrical signalmay represent the amount of artifact present on the selected sensingchannel when INS 26 is delivering neurostimulation therapy, e.g., in theordinary course of neurostimulation therapy. The threshold level may bestored within memory 132 of programmer 24 (FIG. 8), memory 92 of ICD 16,memory 112 of INS 26 or a memory of another device.

In some examples, processor 130 may determine whether the extent ofcrosstalk exceeds a threshold level (302) by comparing a first valueindicative of the difference between a voltage amplitude of the baselineelectrical signal and a sensing threshold of sensing module 96 (FIG. 6)of ICD 16 and a second value indicative of the difference between avoltage amplitude of the second electrical signal and a sensingthreshold of sensing module 96. The relevant voltage amplitudes of thebaseline and second electrical signals may be the average or medianamplitudes over a particular range of time, the amplitudes at aparticular point in time, such as a greatest amplitude over a particularrange of time or a percentage of the greatest amplitude. In addition, insome examples, the voltage amplitude may also comprise an absoluteamplitude value or a root mean square voltage amplitude. In someexamples, the sensing threshold may be the sensing threshold of sensingmodule 96 (FIG. 6) of ICD 16 at the most sensitive setting or at theleast sensitive setting.

If the first and second values do not differ from each other by at leastthe threshold value (or threshold level), processor 130 of programmer 24may determine that the extent of the crosstalk between INS 26 and ICD 16is within an acceptable range. That is, if the difference between thefirst and second is less than or equal to the threshold value, processor130 of programmer 24 may determine that the possibility that ICD 16 maysense the neurostimulation signals delivered by INS 26 andmischaracterize the neurostimulation signals as cardiac signals isrelatively low. Processor 130 may then determine that modifications tothe operating parameters of INS 26 or the sensing parameters of ICD 16are not necessary. Processor 130 of programmer 24 may then continuemeasuring crosstalk (300) and comparing it to a threshold value (302).

On the other hand, if the first and second values differ from each otherby at least the threshold value (or threshold level), processor 130 ofprogrammer 24 may determine that the crosstalk between INS 26 and ICD 16exceeds an acceptable level. In some examples, the threshold level maybe up to about 100% of the sensing threshold of ICD 16, such as about25% to about 50% of the sensing threshold. As previously indicated, thesensing threshold may be the sensing threshold of sensing module 96(FIG. 6) of ICD 16 at the most sensitive setting or at the leastsensitive setting. Thus, in some examples, if the difference between thefirst and second values is greater than the sensing threshold of ICD 16,processor 130 may determine that the crosstalk between INS 26 and ICD 16exceeds an acceptable level. Other percentages or absolute value changesin voltage amplitudes that indicate an unacceptable level ofneurostimulation signal artifact are contemplated.

In other examples, processor 130 may determine whether the extent ofcrosstalk exceeds a threshold level (302) by comparing the spectralcontent of the baseline electrical signal and the second electricalsignal. For example, processor 130 may implement a fast Fouriertransform algorithm in order to extract the frequency components of thebaseline electrical signal and the second electrical signal. Processor130 may compare one or more frequency components of the baselineelectrical signal and the second electrical signal. The one or morefrequency components may include, for example, a power level within oneor more frequency bands, a trend in the power level within one or morefrequency bands over time, a ratio of power levels between one or morefrequency bands, and the like. Different frequency bands may be morerevealing of the extent to which the second electrical signal includesan unacceptable level of neurostimulation signal artifact. A clinicianmay determine the revealing frequency bands during a trial phase inwhich INS 26 and ICD 16 are tested to determine the frequency bands arerelatively revealing of a neurostimulation artifact that adverselyaffects the sensing of cardiac signals by ICD 16.

If processor 130 of programmer 24 determines that the extent of thecrosstalk between INS 26 and ICD 16 exceeds an acceptable level,processor 130 may initiate the modification to one or more stimulationparameter values with which stimulation generator 114 of INS 26generates and delivers neurostimulation therapy to patient 12 or one ormore sensing parameter values of ICD 16 (304). Processor 130 mayinitiate the modification to the one or more stimulation parametervalues of INS 26 using any suitable technique. In one example, processor130 may transmit a control signal to processor 110 of INS 26, andprocessor 110 may initiate the modification to the one or morestimulation parameter values upon receiving the control signal fromprocessor 130 of programmer 24. For example, processor 110 may modifythe one or more stimulation parameter values using a set of rules storedin memory 112, as described with respect to FIGS. 11A-11D. Examples ofstimulation parameter values that processor 110 may modify include, butare not limited to, an electrode combination, voltage amplitude, currentamplitude, pulse rate, pulse duration, and the like. As another example,processor 110 may modify the one or more stimulation parameter values byswitching therapy programs, as described with respect to FIGS. 12A and12B.

In other examples, processor 130 of programmer 24 may provide processor110 of INS 26 with a new therapy program defining one or morestimulation parameter values or provide processor 110 with specificinstructions for modifying the one or more stimulation parameter values.For example, the instructions may indicate that processor 110 of INS 26should decrease the frequency of the neurostimulation signal by acertain percentage or to a specific value. Other types of therapyparameter value modification instructions are contemplated. In otherexamples, processor 130 of programmer 24 may instruct processor 110 ofINS 26 to modify one or more stimulation parameter values by switchingtherapy programs, as described with respect to FIG. 12A.

Processor 130 may initiate the modification to the one or more sensingparameters of ICD 16 using any suitable technique. In one example,processor 130 may transmit a control signal to processor 90 of ICD 16,and processor 90 may initiate the modification to the one or moresensing parameters upon receiving the control signal from processor 130of programmer 24. For example, processor 90 may modify the one or moresensing parameter values by switching sense modes, as described withrespect to FIGS. 16, 19A, and 19B. Examples of sensing parameters valuesthat processor 90 may modify include, but are not limited to, a sensingthreshold value, an amplifier gain, a sensing vector, and a type offilter used by sensing module 96 or processor 90 to filter noise out ofa sensed signal.

After the one or more neurostimulation parameter values or ICD 16sensing parameters are modified (304), processor 130 of programmer 24may measure the crosstalk (306) and determine whether the extent of thecrosstalk exceeds a threshold level (308), e.g., using the techniquesdescribed above. If processor 130 determines that the extent of thecrosstalk does not exceed the threshold level, processor 130 may nottake any further action to modify the one or more neurostimulationparameter values of INS 26. Processor 130 may then continue periodicallyor continuously measuring the crosstalk (300) until a condition in whichthe crosstalk exceeds a threshold level (302) is detected.

On the other hand, if processor 130 determines that the extent of thecrosstalk exceeds the threshold level (308), processor 130 may suspendthe delivery of neurostimulation by INS 26 (310). In other examples,prior to suspending the delivery of neurostimulation, processor 130 mayinitiate the modification to one or more stimulation parameter values ofINS 26 or sensing parameters of ICD 16 in an attempt to minimize theneurostimulation artifact on the signal sensed by ICD 16. Processor 130may repeat the steps shown in blocks 304, 306, and 308 to attempt toreduce the neurostimulation artifact. The one or more stimulationparameter values or sensing parameters may be modified for one or moreiterations prior to suspending the delivery of neurostimulation by INS26. As described with the technique shown in FIGS. 11A-11D, in someexamples, processor 130 may modify a different stimulation parametervalue or sensing parameter during each iteration of the stimulationparameter value modification (304), may modify the same stimulationparameter or sensing parameter for at least two consecutive ornonconsecutive iterations or may modify more than one type ofstimulation parameter value in the same iteration of INS 26modification.

Processor 130 of programmer 24 may generate an interference indicationif the extent of the crosstalk between INS 26 and ICD 16 exceeds athreshold level, despite the modification to one or more stimulationparameter values (312). Processor 130 may present the interferenceindication to a user (e.g., a clinician or patient 12) via a displayuser interface 134 or processor 130 may generate an audible or asomatosensory alert (e.g., a pulse vibration of programmer 24) viaprogrammer 24. In this way, programmer 24 may present a real-timeinterference alert to a user to notify the user that the stimulationdelivered by INS 26 may be interfering with the sensing of cardiacsignals by ICD 16.

In some examples, a characteristic of the visual, auditory orsomatosensory alert may change in response to the amount of crosstalkdetermined to exist between ICD 16 and INS 26. For example, if thevisual alert includes displaying a colored display, the color of thedisplay may change or change intensity as a function of the amount ofcrosstalk determined to exist between ICD 16 and INS 26. As anotherexample, if the interference indication comprises an audible alert, thetone, frequency, volume or another characteristic of the audible soundmay change as a function of the amount of crosstalk determined to existbetween ICD 16 and INS 26. The amount of crosstalk determined to existbetween ICD 16 and INS 26 may be based on a difference between the firstand second values, where the first value is indicative of the differencebetween the characteristic of the baseline electrical signal and thesensing threshold of ICD 16 and the second value is indicative of thedifference between the characteristic of the second electrical signaland the sensing threshold of ICD 16. For example, processor 130 maydetermine that the greater the difference between the first and secondvalues, the more crosstalk is present between ICD 16 and INS 26.

The interference indication may also indicate that the delivery ofneurostimulation by INS 26 was adjusted (e.g., suspended or theintensity of neurostimulation was reduced) and or that patient 12 shouldseek medical attention. As previously indicated, the tonal frequency ofthe audible alert or the pulse rate or intensity of the somatosensoryalert may change as a function of the relative level of crosstalkbetween INS 26 and ICD 16. For example, the intensity of thesomatosensory alert or the pitch of the audible alert may change withthe strength of the neurostimulation artifact present in the signalsensed by ICD 16.

In some examples, processor 130 may transmit the interference indicationto a remote site, such as a remote clinician's office, via a network, asdescribed with respect to FIG. 32. In addition, in some examples,processor 130 may also store the interference indication in memory 132.The interference indication may indicate, e.g., to a clinician, that thecrosstalk between INS 26 and ICD 16 was not reducible by modifying oneor more stimulation parameter of INS 26 or one or more sensingparameters of ICD 16. After receiving the interference indication, theclinician may determine whether other measures may be taken in order toreduce the crosstalk between INS 26 and ICD 16. For example, theclinician may determine whether repositioning lead 28 coupled to INS 26within patient 12 may help reduce the crosstalk.

In other examples of the technique shown in FIG. 21, as well as FIGS. 23and 24, processor 90 of ICD 16 or processor 110 of INS 26 may performany part of the technique shown in FIG. 21 in addition to or instead ofprocessor 130 of programmer 24.

In other examples of the technique shown in FIG. 21, processor 130 mayevaluate the extent of crosstalk between ICD 16 and INS 26 based only onthe second electrical signal, which is sensed by ICD 16 during deliveryof neurostimulation by INS 26. For example, rather than comparing thebaseline and second electrical signals to determine whether the extentof crosstalk exceeds an acceptable level (302), processor 130 maydetermine that if the difference between an amplitude of the secondelectrical signal and a sensing threshold of sensing module 96 during aquiet segment of cardiac cycle of heart 14 (FIG. 1) of patient 12 isgreater than or equal to a stored value, the noise on the sensingchannel of sensing module 96 is greater than an acceptable level. Theamplitude may be a mean or median amplitude, a highest amplitude, a RMSamplitude, an amplitude that is equal to a certain percentage (e.g.,about 95%) of the highest amplitude, and the like. The stored value maybe a percentage of the sensing threshold of sensing module 96 of ICD 16.For example, the stored value may be about 10% to about 50%, such asabout 25% of the sensing threshold voltage.

The noise on the sensing channel of sensing module 96 may be at leastpartially attributable to the delivery of neurostimulation by INS 26. Inthis way, a comparison of a stored value and the difference between anamplitude of the second electrical signal and a sensing threshold ofsensing module 96 may indicate whether the extent of crosstalk betweenICD 16 and INS 26 is undesirable.

FIG. 22 is a conceptual illustration of programmer 24, which may displaya status level of the INS 26 and ICD 16 interference. The interferencestatus may be referred to as, for example, an electrical noise status orcrosstalk status. In the example shown in FIG. 22, programmer 24includes user input mechanisms 320A-320G (collectively “user inputmechanisms 320”) and display 322. A user (e.g., patient 12 or aclinician) may interact with user input mechanisms 320 to inputinformation into programmer 24, and, in some cases, control aspects oftherapy delivered by ICD 16 and/or INS 26 within the limits programmedby a clinician. User input mechanisms 320 include buttons 320A and 320B,which may be used to increase or decrease the therapy intensitydelivered by INS 26, if allowed, and may perform other functions. Anintensity of therapy may be modified by, for example, modifying atherapy parameter value, such as the current or voltage amplitude ofstimulation signals, the frequency of stimulation signals, the shape ofa stimulation signal or the electrode combination used to deliver thestimulation signal. In some examples, user input mechanisms 320C, 320Dmay be used to decrease or increase the contrast of display 322, anduser input mechanism 320E may be used to power programmer 24 on and off.

Multi-directional controller 320F may allow a user to navigate throughmenus displayed by display 322, and may include a button 320G that isactuated when the center of multi-directional controller 320F ispressed. Display 322 may comprise any suitable type of display, such asan LCD display, LED display or a touch screen display. Display 322 maypresent graphical user interface screens for presenting information tothe user, such as information related to the sensed level ofneurostimulation signal artifact on a selected sense channel of ICD 16.In the example shown in FIG. 21, display 322 presents first screen 324that indicates crosstalk status, a second screen 328 that illustrates awaveform of a baseline electrical signal that is sensed on the selectedsense channel of ICD 16 when INS 26 is not delivering electricalstimulation to patient 12, and a third screen 330 that illustrates awaveform of a second electrical signal that is sensed on the selectedsense channel of ICD 16 when INS 26 is delivering electrical stimulationto patient 12.

The user may review the different waveforms present in screens 328, 330in order to visually ascertain the extent to which the neurostimulationartifact on the selected sensing channel of ICD 16 may be affecting thedetection of true cardiac signals. Status screen 324 presents anindication of whether the neurostimulation signal artifact exceeds athreshold level or whether the stimulation signal artifact issufficiently low, such that the neurostimulation signal delivered by INS26 does not adversely affect the sensing of cardiac signals by ICD 16.In the example shown in FIG. 22, status screen 324 provides anindication that ICD 16 may be oversensing cardiac signals, i.e., theneurostimulation artifact on the selected sensing channel of sensingmodule 96 of ICD 16 exceeds a threshold level.

In other examples, programmer 24 may present other types of displays toprovide information to a user regarding the neurostimulation signalartifact on one or more sensing channels of ICD 16. For example, in someexamples, processor 130 of programmer 24 may categorize aneurostimulation signal artifact based on the probability that theartifact will affect the sensing of true cardiac signals by ICD 16. Thecategorization of the neurostimulation signal artifact maybe useful forproviding a relatively quick and easy way to ascertain the extent ofcrosstalk between INS 26 and ICD 16.

FIG. 23 is a flow diagram illustrating an example technique forcategorizing a neurostimulation signal artifact. Processor 130 ofprogrammer 24 may measure the extent of the crosstalk between INS 26 andICD 16 (300), e.g., by determining a difference between a characteristicof a baseline electrical signal and a respective characteristic of thesecond electrical signal sensed by ICD 16 while INS 26 is deliveringstimulation, as described with respect to FIG. 21. Processor 130 maydetermine a difference between the characteristics of the baseline andsecond electrical signals using any suitable technique. In someexamples, processor 130 determines a difference between thecharacteristics of the baseline and second electrical signals bydetermining a difference between a first value indicative of thedifference between an amplitude of the baseline electrical signal and asensing threshold value of sensing module 96 (FIG. 6) of ICD 16 and asecond value indicative of the difference between an amplitude of thesecond electrical signal and the sensing threshold value of sensingmodule 96.

Processor 130 may determine whether the characteristics of baseline andsecond electrical signals differ by a first threshold value (340). Asdiscussed with respect to FIG. 21, in some examples, the threshold valuemay be based on the sensing threshold of ICD 16, e.g., may be less thanthe sensing threshold, such as about 1% to about 99% of the sensingthreshold or about 25% to about 50% of the sensing threshold amplitude.In the example shown in FIG. 23, memory 132 of programmer 24 stores aplurality of threshold values (or threshold levels) that are eachassociated with a different neurostimulation signal artifact category.The different categories may represent the relative intensity of theneurostimulation artifact on a selected sense channel of ICD 16. Thethreshold values may be adjustable. For example, a clinician may programthe threshold values into programmer 24, ICD 16, INS 26 or anotherdevice. The threshold values may be specific to a particular patient.

If processor 130 determines that the characteristics of baseline andsecond electrical signals do not differ by at least the first thresholdvalue, processor 130 may determine that the extent of the crosstalkbetween INS 26 and ICD 16 falls within a first category, and processor130 may generate a category one indication (342). The first category ofcrosstalk may be associated with a crosstalk level in which crosstalkbetween INS 26 and ICD 16 is present, but the extent of the crosstalk isrelatively low. Processor 130 may determine that modifications to one ormore stimulation parameters of INS 26 or one or more sense parameters ofICD 16 are not necessary when a category one indication is generated.

If processor 130 determines that the characteristics of the baseline andsecond electrical signals differ by at least the first threshold value,processor 130 may determine whether the characteristics of the baselineand second electrical signals differ by a second threshold level that isdifferent than the first threshold level (344). In some examples, thesecond threshold level may be associated with a greater artifactintensity than the first threshold level. For example, the firstthreshold value may include a first voltage amplitude value or a firstpercentage that indicates a percentages change of a voltage amplitude ofthe second electrical signal relative to a baseline electrical signal.The second threshold level may include a second voltage amplitude valueor a second percentage, where the second voltage amplitude value orpercentage are greater than the first voltage amplitude value orpercentage, respectively.

If processor 130 determines that the characteristics of the baseline andsecond electrical signals do not differ by at least the second thresholdvalue, processor 130 may determine that the extent of the crosstalkbetween INS 26 and ICD 16 is within a second category, and processor 130may generate a category two indication (346). In some examples, thesecond category of crosstalk may be associated with a crosstalk level inwhich crosstalk between INS 26 and ICD 16 exceeds an acceptable level.Thus, as shown in FIG. 23, upon generating the category two indication,processor 130 may initiate the modification to one or more stimulationparameter values of INS 26 or one or more sensing parameters of ICD 16(304).

If processor 130 determines that the characteristics of the baseline andsecond electrical signals differ by at least the second threshold level,processor 130 may determine whether the characteristics of the baselineand second electrical signals differ by a third threshold value that isdifferent than the first and second threshold values (348). In someexamples, the third threshold value may be associated with a greaterartifact intensity than the first and second threshold levels. Forexample, the third threshold level may include a third voltage amplitudevalue or a third percentage, where the third voltage amplitude value orpercentage are greater than the first and second voltage amplitudevalues or percentages, respectively.

If processor 130 determines that the characteristics of the baseline andsecond electrical signals do not differ by at least the third thresholdvalue, processor 130 may determine that the extent of the crosstalkbetween INS 26 and ICD 16 is within the second category, and processor130 may generate a category two indication (346). On the other hand, ifprocessor 130 determines that the characteristics of the baseline andsecond electrical signals differ by at least the third threshold value(348), processor 130 may generate a category three indication (350). Insome examples, the third category of crosstalk may be associated with acrosstalk level in which crosstalk between INS 26 and ICD 16 exceeds anacceptable level. Thus, as shown in FIG. 23, upon generating thecategory three indication, processor 130 may initiate the modificationto one or more stimulation parameter values of INS 26 or one or moresensing parameters of ICD 16 (304). These modifications may be the sameor different as the modifications made in response to the generation ofa category two indication (346). In addition, in some examples, themodification to the one or more stimulation parameter values of INS 26may result in the suspension of the delivery of neurostimulation by INS26 upon generation of the category three indication.

In some examples, processor 130 of programmer 24 or a processor ofanother device may evaluate the extent of crosstalk between ICD 16 andINS 26 based on the difference between one or more characteristics ofthe baseline and second electrical signals during a quiet segment of acardiac cycle of heart 14. As previously indicated the second electricalsignal may be the electrical signal sensed by ICD 16 on a particularsense channel while INS 26 delivers neurostimulation signals to patient12. The quiet segment of a cardiac cycle may be when the intrinsicelectrical signal of heart 14 is least active, such as during the T-Psegment of a sinus rhythm of heart 14. Because the absolute value of avoltage amplitude of a true cardiac signal may be the lowest during thequiet segment, determining a voltage amplitude of a second electricalsignal sensed by ICD 16 on a particular sensing channel during the quietsegment may provide a more useful indication of the artifact present onthe sensing channel of ICD 16. The difference in voltage amplitudesbetween a baseline signal and a second electrical signal during thequiet segment may be more pronounced and, therefore, more revealing ofthe crosstalk between ICD 16 and INS 26.

FIG. 24 is a flow diagram illustrating an example technique for parsingdata from a baseline electrical signal and the second electrical signalthat is sensed by the selected sense channel of ICD 16 during activedelivery of stimulation by INS 26. The parsed data may indicate thevoltage amplitude of the baseline signal or the second electrical signalsensed by ICD 16 during a quiet segment of a cardiac cycle of heart 14.Processor 130 of programmer 24 may receive a cardiac signal thatincludes a plurality of cardiac cycles (360), such as about 10 cardiaccycles to about 20 cardiac cycles. A cardiac cycle may be defined by,for example, a sinus rhythm including a QRST segment.

Processor 130 may identify the portion of the received electricalcardiac signals that correspond to the quiet segment of each cardiaccycle (362). As previously indicated, in some examples, the quietsegment may include the T-P segment of a sinus rhythm. Processor 130 maydigitize the portions of the cardiac signals corresponding to the quietsegments (364), e.g., defining each quiet segment as about six points,although any suitable number of digitized points may be used.

Processor 130 may convert the digitized quiet segment portions of thecardiac cycles into a waveform in order to determine the peak-to-peakvoltage amplitude (V_(PK-PK)) (366). Processor 130 may filter the directcurrent (DC) component out of the waveform in order to removelow-frequency artifact prior to determining the root mean square (RMS)amplitude of the waveform indicative of the quiet segment (368).Processor 130 may determine the mean and median peak-to-peak voltageamplitudes (V_(PK-PK)) (370), and determine the mean and median rootmean square amplitudes (V_(RMS)) of the waveform indicative of the quietsegment of the cardiac signal based on the mean and median peak-to-peakvoltage amplitudes (372). For example, processor 130 may determine themean root mean square amplitude by determining the square root of thesquare of the mean peak-to-peak voltage amplitudes.

In order to evaluate the extent of crosstalk between ICD 16 and INS 26,processor 130 may compare the RMS voltage amplitudes of the baseline andsecond electrical signals and determine whether the RMS amplitudesdiffer by one or more threshold values, as generally described withrespect to FIG. 21.

In some examples, the extent of the crosstalk between INS 26 and ICD 16may be evaluated based on one or more characteristics of an electricalsignal that is sensed by ICD 16 when INS 26 is delivering an electricalsignal that does not provide any therapeutic benefits to patient 12. Forexample, INS 26 may generate and deliver a test electrical signal thatdoes not provide stimulation therapy to patient 12, and ICD 16 may senseelectrical cardiac signals while INS 26 is delivering the test signals.In some examples, patient 12 does not perceive the test electricalsignal, due to, for example, the intensity of the test signal and/or thetiming of delivery of the test signal. For example, test electricalsignal may comprise a sub-threshold amplitude signal that does notcapture or otherwise activate tissue (e.g., neurons within the tissue)of patient 12. An intensity of stimulation may be modified by modifyingthe current or voltage amplitude of a stimulation signal, a frequency ofthe stimulation signal, and, if the signal comprises a pulse, a pulsewidth or pulse shape of the stimulation signal.

FIG. 25 is a flow diagram illustrating an example technique fordetermining an extent of crosstalk between INS 26 and ICD 16 with a testsignal that does not provides little to no therapeutic benefits topatient 12. In the example shown in FIG. 25, processor 110 of INS 26 maycontrol signal generator 114 to generate and deliver a test signal topatient 12 (373). The test signal may be nontherapeutic, e.g., does notprovide efficacious therapy to patient 12 or provides minimallyefficacious therapy to patient 12. In contrast, a therapeutic electricalstimulation signals delivered by INS 26 may have a greater voltageamplitude, current amplitude, frequency or a different burst patternthan the test signal delivered by INS 26. Memory 112 of INS 26, memory132 of programmer 24 or a memory of another device may store a therapyprogram that defines the signal parameter values for the test signal. Inaddition, in some examples, the test signal may comprise an amplitudethat is less than an activation threshold of tissue, such that thepatient's tissue is not substantially affected by the delivery of thetest signal. Furthermore, in some examples, the test signal may comprisean amplitude that is less than a perception threshold of patient 12,such that patient 12 does not perceive the delivery of the test signalby INS 26.

As INS 26 generates and delivers the test signal, sensing module 96(FIG. 6) of ICD 16 may sense an electrical signal via a selected sensingchannel (374), although more than one sensing channel may also be usedin other examples. Processor 90 of ICD 16 may determine whether acharacteristic of the sensed electrical signal exceeds a threshold value(376). The threshold value may indicate an amplitude value at which theelectrical signal sensed by the selected sensing channel of ICD 16indicates that the extent of crosstalk between INS 26 and ICD 16 mayexceed an acceptable level if INS 26 delivers neurostimulation signalsin an ordinary course, e.g., according to a therapy program definingtherapeutic neurostimulation signals. While the delivery of the testsignal by INS 26 may not result in an unacceptable level of crosstalkbetween INS 26 and ICD 16, one or more characteristics of the signalthat is sensed by ICD 16 during the delivery of the test signal by INS26 may be represent a neurostimulation artifact that may result if INS26 delivers neurostimulation signals in an ordinary course.

A clinician may determine the threshold value using any suitabletechnique. In one example, the clinician may detect when there is anunacceptable level of crosstalk between INS 26 and ICD 16, e.g., basedon actual signals sensed by ICD 16 when INS 26 delivers therapeuticneurostimulation signals to patient 12. Shortly thereafter, e.g., whileleads 28, 29 are likely in the same position as when the unacceptablelevel of crosstalk was detected, the clinician may control INS 26 todeliver the test signal to patient 12. The electrical signal that issensed by ICD 16 while INS 26 delivers the test signal to patient 12 maybe indicative of the unacceptable level of crosstalk between INS 26 andICD 16. Thus, one or more characteristics of the electrical signal thatis sensed by ICD 16 while INS 26 delivers the test signal to patient 12may be stored as a threshold value, e.g., in memory 92 of ICD 16 ormemory 112 of INS 26.

Determining the extent of potential crosstalk between INS 26 and ICD 16prior to delivering therapeutic neurostimulation therapy to patient 12may be useful for confirming that the extent of crosstalk between INS 26and ICD 16 is within an acceptable range in advance of delivering theneurostimulation therapy. This may help mitigate the possibility thatthe delivery of neurostimulation by INS 26 interferes with the sensingof cardiac signals by ICD 16.

If processor 90 of ICD 16 determines that one or more characteristics ofthe sensed electrical signal is greater than or equal to the thresholdvalue (376), processor 90 may suspend the delivery of therapeuticelectrical stimulation by INS 26 (378). If processor 90 of ICD 16determines that the sensed electrical signal does not exceed thethreshold value (376), processor 90 may determine that the relativelevel of crosstalk between INS 26 and ICD 16 is within an acceptablelevel. Processor 90 may then provide INS 26 with a controls signal thatindicates that INS 26 may generate and deliver therapeutic electricalstimulation to patient 12 (380).

The technique shown in FIG. 25 may be implemented to evaluate the extentof crosstalk between INS 26 and ICD 16 at any suitable evaluationfrequency. In some examples, INS 26 may deliver the test signal topatient 12 (373) at a test frequency of about one to about ten times perminute, although more frequent (e.g., about 1 Hz to about 100 Hz) orless frequent testing frequencies are contemplated. INS 26 may notifyICD 16 prior to sending the test signal or ICD 16 and INS 26 may havesynchronized clocks such that ICD 16 senses the electrical signal on aselected sensing channel (374) at substantially the same time that INS26 delivers the test signal.

In some cases, the one or more characteristics of the electrical signalsensed by ICD 16 while INS 26 is delivering the non-therapeutic testsignal may also indicate an intensity of stimulation signals that INS 26may deliver without adversely affecting the sensing of cardiac signalsby ICD 16. For example, the one or more characteristics of theelectrical signal (e.g., a voltage or current amplitude) may beassociated with a specific therapy program or instructions for modifyinga therapy program in memory 132 (FIG. 8) of programmer 24, memory 112(FIG. 7) of INS 26 or memory 92 (FIG. 6) of ICD 16.

Processor 130 of programmer 24 or another device may determine the oneor more characteristics of the electrical signal sensed by ICD 16 whileINS 26 is delivering the non-therapeutic test signal to patient 12. Ifthe one or more characteristics of the signal are less than thethreshold value (376), thereby indicating that the crosstalk between ICD16 and INS 26 is acceptable, processor 130 may determine an acceptablestimulation therapy program for INS 26. For example, processor 130 mayreference a data structure stored in memory 112 to determine the therapyprogram or instructions for modifying a therapy program. Processor 130may then instruct processor 110 of INS 26 to deliver therapy to patient12 in accordance with the therapy program associated with the one ormore characteristics of the electrical signal or in accordance withtherapy parameters modified based on the instructions associated withthe one or more characteristics of the electrical signal.

The therapy programs or instructions for modifying a therapy programbased on the one or more characteristics of the electrical signal sensedby ICD 16 while INS 26 is delivering the non-therapeutic test signal topatient 12 may be determined during a programming session with aclinician. The clinician may determine a characteristic of an electricalsignal sensed by ICD 16 while INS 26 is delivering the non-therapeutictest signal to patient 12, and determine the therapy parameter valuesthat provide efficacious therapy to patient 12 without interfering withthe sensing of cardiac signals by ICD 16. These therapy parameter valuesmay then be associated with the signal characteristic in memory 132 (ora memory of another device) as a therapy program or an instruction formodifying a baseline therapy program.

The delivery of electrical stimulation by INS 26 may change an amplitudeof an electrical cardiac signal (e.g., an EGM) sensed by ICD 16. Thus,in some examples, the crosstalk status of a therapy system including ICD16 and INS 26 may be evaluated based on a change in amplitude of anelectrical cardiac signal sensed while INS 26 is not actively deliveringstimulation to patient 12 and an electrical cardiac signal sensed whileINS 26 is delivering stimulation to patient 12.

FIG. 26 illustrates a flow diagram of an example technique fordetermining a crosstalk status (or an electrical noise status) oftherapy system 10. In the technique shown in FIG. 26 Processor 90 of ICD16 may instruct processor 110 of INS 26 to suspend or otherwise adjustthe delivery of neurostimulation (381). For example, processor 90 maytransmit a control signal to processor 110 via the respective telemetrymodules 98 (FIG. 6), 118 (FIG. 7). The control signal may not onlyindicate whether INS 26 should suspend or otherwise adjust the deliveryof neurostimulation to patient 12, but, in some examples, may indicatehow long INS 26 should suspend neurostimulation or deliver therapyaccording to the adjusted parameters. In other examples, memory 112(FIG. 7) of INS 26 may store instructions for suspending or otherwiseadjusting neurostimulation when processor 110 of INS 26 receives thecontrol signal from processor 90 of ICD 16. As another example, INS 26may suspend or otherwise adjust delivery of stimulation withoutintervention from ICD 16, e.g., according to schedule stored by memory112.

During the time in which neurostimulation is suspended or adjusted,sensing module 96 (FIG. 6) of ICD 16 may sense a first electricalcardiac signal and processor 90 may determine a first characteristic ofthe first electrical cardiac signal (382). In some examples, the firstcharacteristic may be a mean or median P-wave or R-wave amplitude over apredetermined period of time. Processor 90 of ICD 16 may then activatethe delivery of stimulation by INS 26 (384). For example, processor 90may generate a control signal that is transmitted to processor 110 ofINS 26 via the respective telemetry modules 98 (FIG. 6), 118 (FIG. 7).Upon receiving the control signal, processor 110 of INS 26 may controlstimulation generator 114 to begin generating and deliveringneurostimulation therapy. In other examples, processor 110 of INS 26 maybegin generating and delivering neurostimulation therapy based on apredetermined schedule that indicates the times at which processor 110should suspend the delivery of neurostimulation and initiate thedelivery of stimulation.

After INS 26 commences the delivery of neurostimulation to patient 12,processor 90 may control sensing module 96 to sense a second electricalcardiac signal of heart 14 of patient 12. Processor 90 may determine asecond characteristic of the second electrical cardiac signal (386). Insome examples, the first and second characteristics may be similarcharacteristics. For example, the first and second characteristics maybe a mean or median P-wave or R-wave amplitude of the first and secondelectrical cardiac signals, respectively, over a predetermined period oftime.

Processor 90 may determine whether the first and second characteristicsare within a threshold range of each other (388). In general, if thefirst and second characteristics are similar, e.g., within a thresholdrange of each other, the crosstalk status of the therapy systemincluding ICD 16 and INS 26 may be relatively low. The threshold rangemay be, for example, about 20% of the value of the first characteristic,such as about 5% to about 20%, about 10% to about 15%, or substantiallyequal. Thus, in some examples, if the difference between the first andsecond characteristics is less than about 20% of the value of the firstcharacteristic, processor 90 may determine that the first and secondcharacteristics are within a threshold range of each other.

First and second characteristics that are within a threshold range ofeach other may indicate that the delivery of neurostimulation by INS 26has a minimal affect on the electrical cardiac signal sensed by ICD 16,such that the possibility that ICD 16 may sense the neurostimulationsignal and mischaracterize the signal as an electrical cardiac signal isrelatively low. In such a situation, the crosstalk status may beacceptable.

If the first and second characteristics are within a threshold range ofeach other, processor 90 may continue comparing the first and secondcharacteristics of subsequently sensed electrical cardiac signals inaccordance with the technique shown in FIG. 26. On the other hand, ifthe first and second characteristics are not within a threshold range ofeach other, processor 90 may determine that the extent of crosstalkbetween ICD 16 and INS 26 is unacceptable, e.g., that the crosstalkstatus is unacceptable. Accordingly, processor 90 may generate acrosstalk indication (389) if the first and second characteristics arenot within a threshold range of each other. The crosstalk indication maybe a value, flag, or signal that is stored or transmitted to indicatethe unacceptable crosstalk status. In some examples, processor 90 or 110may transmit the crosstalk indication to programmer 24 or anotherexternal device, including remote devices, e.g., using a systemdescribed with respect to FIG. 32. In some examples, programmer 24 maypresent a notification to a user via user interface 134 (FIG. 8) toindicate an unacceptable level of crosstalk was detected.

While FIG. 26 is described with respect to processor 90 of ICD 16, inother examples, processor 130 of programmer 24 or processor 110 of INS26 or another device may perform any part of the technique shown in FIG.26. For example, a clinician may evaluate the crosstalk status betweenICD 16 and INS 26 with the aid of programmer 24. Processor 130 ofprogrammer 24 may perform any part of the technique shown in FIG. 26.For example, processor 130 may determine the first and secondcharacteristics (382, 386) based on electrical cardiac signals sensed byICD 16 and transmitted to programmer 24 by ICD 16.

In some examples, ICD 16 and/or INS 26 may periodically check theimpedance of one or more electrical paths, each path comprising two ormore implanted electrodes on one or more implanted leads. For example,processor 90 of ICD 16 may initiate a check of the impedance of anelectrical path comprising lead 18 (FIG. 3) and electrodes 50, 52, 72.ICD 16 and/or INS 26 may, for example, check the impedance of one ormore electrical paths comprising an electrode prior to deliveringelectrical stimulation to patient 12 in order to confirm that electricalinterference or lead-related conditions that may affect the efficacy ofthe delivery of stimulation to patient 12 are not present.

The impedance measurements may be used to detect lead-relatedconditions, such as short circuits, open circuits or significant changesin impedance that may adversely affect the performance of therapydelivery by ICD 16 or INS 26 or sensing by ICD 16 or INS 26. Changes inimpedance of an electrical path that is electrically connected to ICD 16or INS 26 may increase the amount of crosstalk observed by ICD 16 by,for example, effectively widening a stimulation dipole of INS 26 or asensing dipole of ICD 16 by creating a leakage path due to alead-related condition, such as a lead fracture. A lead-relatedcondition my often cause noise on a sensing channel of ICD 16. Thus, thetechnique shown in FIG. 25 may be useful for identifying a lead-relatedcondition.

In some examples, lead integrity testing may also involve comparing themeasured impedance to a threshold in order to determine whether thelead(s) have a lead-related condition. This integrity testing may beperformed periodically, e.g., while patient 12 is sleeping or as patient12 moves and subjects any of the leads 18, 20, 22, 28, 29 coupled to ICD16 or INS 26 to mechanical stresses.

ICD 16 and INS 26 may measure impedance by determining an electricalparameter value indicative of the impedance. In some examples, ICD 16 orINS 26 may perform an impedance measurement by delivering, from therespective stimulation generator 94, 114, an electrical signal having aconstant voltage between at least two electrodes, and measuring aresulting current of the signal that is sensed by two or moreelectrodes. The respective processor 90, 110 may determine a resistancebased upon the voltage amplitude of the electrical signal and themeasured amplitude of the resulting current. The current of the sensedsignal or the determined resistance may be electrical parameter valuesindicative of the impedance path comprising the electrodes.

In other examples, ICD 16 or INS 26 may perform impedance measurement bydelivering, from the respective stimulation generator 94, 114, a currentpulse across at least two electrodes, and measuring a resulting voltageof a signal that is sensed by two or more electrodes. The respectiveprocessor 90, 110 may determine a resistance based upon the currentamplitude of the pulse and the measured amplitude of the resultingvoltage. The voltage of the sensed signal or the determined resistancemay be electrical parameter values indicative of the impedance pathcomprising the electrodes.

Sensing module 96 of ICD 16 and a sensing module of INS 26 may includecircuitry for measuring amplitudes of resulting currents or voltages,such as sample and hold circuitry. ICD 16 and INS 26 may use defined orpredetermined pulse amplitudes, widths, frequencies, or electrodepolarities for the pulses delivered for these various impedancemeasurements. In these examples, stimulation generators 94, 114 maydeliver electrical signals that do not necessarily deliver stimulationtherapy to patient 12, due to, for example, the amplitudes of suchsignals and/or the timing of delivery of such signals. For example,these signals may comprise sub-threshold amplitude signals that may notstimulate tissue, e.g., below a threshold necessary to capture orotherwise activate tissue. In the case of ICD 16, the electrical signalsfor measuring impedance of an electrical path may be delivered during arefractory period, in which case they also may not stimulate heart 14.

In certain cases, ICD 16 and INS 26 may collect electrical parametervalues that include both a resistive and a reactive (i.e., phase)component. In such cases, ICD 16 and INS 26 may measure impedance duringdelivery of a sinusoidal or other time varying signal by the respectivestimulation generator 94, 114. Thus, as used herein, the term“impedance” is used in a broad sense to indicate any collected,measured, and/or determined value that may include one or both ofresistive and reactive components. Impedance data may include electricalparameter values that can be used to determine impedance (such ascurrent and/or voltage values).

Crosstalk between INS 26 and ICD 16 may adversely affect the impedancemeasurements take by ICD 16 and INS 26. For example, the electricalstimulation signals generated and delivered by INS 26 may be sensed byICD 16 during a bipolar, tripolar or quadrapolar impedance measurement.Similarly, the electrical stimulation signals (e.g., pacing pulses ordefibrillation pulses) generated and delivered by ICD 16 may be sensedby INS 26 during a bipolar, tripolar or quadrapolar impedancemeasurement. Inaccurate impedance measurements by either INS 26 or ICD16 may adversely affect the system integrity checks performed by INS 26or ICD 16, such as by causing ICD 16 or INS 26 to over-sense orunder-sense a system integrity issue. Oversensing a system integrityissue may be undesirable because of, for example, the time required forpatient 12 to resolve a false-positive system integrity issue.Undersensing a system integrity issue may also be undesirable because asystem integrity issue may affect the efficacy of therapy delivery byICD 16 and INS 26, and, therefore, it may be desirable for systemintegrity issues to be addressed by qualified individual as soon aspossible.

FIG. 27 is a flow diagram of an example technique that may beimplemented in order to determine whether the crosstalk between ICD 16and INS 26 may be adversely affecting the impedance measurements takenby ICD 16. Processor 90 of ICD 16 may control INS 26 to suspend orotherwise adjust (e.g., decrease the intensity) the delivery ofneurostimulation (290), as described above with respect to FIG. 20.Processor 90 may determine a first electrical parameter value indicativeof an impedance of an electrical path (390), e.g., by delivering avoltage pulse or a current pulse and determining a resulting current orvoltage, respectively. Thereafter, processor 90 may activate thedelivery of neurostimulation signals by INS 26, e.g., as described abovewith respect to FIG. 20 (294).

While INS 26 is delivering neurostimulation signals to patient 12,processor 90 of ICD 16 may determine a second electrical parameter valueindicative of the impedance of the electrical path (392). Processor 90may compare the first and second electrical parameter values (394). Ifthe first and second determined impedance values are within a thresholdrange, e.g., within about 20% or less of each other, such as about 5% toabout 20%, about 10% to about 15%, or substantially equal, processor 90may determine that the delivery of neurostimulation by INS 26 does notadversely affect the impedance measurement by ICD 16. Processor 90 mayperiodically perform the technique shown in FIG. 27, such as at animpedance sampling frequency of about 1 Hz to about 100 Hz. Otherfrequencies are contemplated, such as a frequency of about one to aboutten times per minute. In other examples, processor 90 may compare thefirst and second electrical parameter values indicative of impedance by,for example, comparing the difference between the first and secondelectrical parameter values to a threshold value.

If the difference exceeds a threshold value or falls outside of athreshold range of values, processor 90 may determine that the first andsecond electrical parameter values are not within the threshold range ofeach other. If the first and second determined impedance values are notwithin the threshold range of each other (394), processor 90 maygenerate an impedance measurement interference indication (396). Theimpedance measurement interference indication may be a value, flag, orsignal that is stored in memory 92 of ICD 16 or transmitted to anotherdevice (e.g., programmer 24 or INS 26) to indicate that the delivery ofneurostimulation by INS 26 may potentially be interfering with theaccurate and precise impedance measurements of one or more electricalpaths coupled to ICD 16. In some cases, the change in impedance afterINS 26 begins delivering stimulation to patient 12 may also indicatethat a therapy system integrity issue is present, such as a lead-relatedcondition (e.g., a lead fracture). The lead-related condition may berelated to the integrity of one or more of the leads 18, 20, 22 (FIG. 3)electrically connected to ICD 16 or one or more of the leads 28, 29(FIG. 5) electrically connected to ICD 16.

In some examples, processor 90 may initiate the modification to one ormore stimulation parameter values that define the neurostimulationdelivered by INS 26 or suspend the delivery of neurostimulation by INS26 if an impedance measurement interference indication determination isgenerated. FIG. 28 is a flow diagram illustrating an example techniquethat may be implemented to modify the neurostimulation signal deliveredby INS 26 in an attempt to mitigate the effect on impedance measurementsof electrical paths taken by ICD 16. The example technique shown in FIG.28 is substantially similar to the technique shown in FIG. 27. However,after generating the impedance measurement interference indication(396), processor 90 of ICD 16 may initiate the modification to one ormore one or more neurostimulation parameter values (398). For example,processor 90 may instruct processor 110 of INS 26 to modify the one ormore stimulation parameter values or switch therapy programs, orprocessor 90 of ICD 16 may transmit the modified stimulation parametervalues to INS 26.

After the one or more stimulation parameter values are modified,processor 90 may suspend or otherwise adjust the delivery ofneurostimulation by INS 26 (290), determine a first electrical parametervalue indicative of an impedance an electrical path (390), activate thedelivery of neurostimulation by INS 26 (376), determine a secondelectrical parameter value indicative of the impedance of the electricalpath (392), and determine whether the first and second electricalparameter values are within an threshold range of each other (394).Processor 90 of ICD 16 or processor 110 of INS 26 may continue modifyingthe INS 26 stimulation parameter values until processor 90 determinesthat the impedance measurement by ICD 16 is not substantially affectedby the delivery of neurostimulation by INS 26 or until no furtherneurostimulation parameter values may be modified, i.e., all permissibleneurostimulation modifications have been exhausted. The permissibleneurostimulation modifications may set forth ranges for the differentstimulation parameter values that provide efficacious therapy to patient12. Thus, modifying the neurostimulation parameters such that the valuesfall outside of the ranges may result in neurostimulation signals thatdo not provide efficacious therapy to patient 12.

In some examples, the delivery of electrical stimulation, e.g., pacingpulses or defibrillation pulses, by ICD 16 may adversely affectimpedance determinations by INS 26. FIG. 29 is a flow diagramillustrating an example technique for determining whether the deliveryof electrical stimulation by ICD 16 adversely affects impedancedeterminations by INS 26. The technique shown in FIG. 29 is similar tothe technique that may be implemented by ICD 16 and shown in FIG. 27.

Processor 110 of INS 26 may cause ICD 16 to suspend or otherwise adjustthe delivery of stimulation (400), which may include, for example, acardiac rhythm therapy. For example, processor 110 may transmit acontrol signal to processor 90 of ICD 16 via the respective telemetrymodules 118 (FIG. 7), 98 (FIG. 6). The control signal may not onlyindicate whether ICD 16 should suspend the delivery of stimulation topatient 12, but, in some examples, may indicate how long ICD 16 shouldsuspend stimulation. In other examples, memory 92 of ICD 16 may storeinstructions for suspending stimulation when processor 90 receives thecontrol signal from processor 110 of INS 26. As another example, ICD 16may suspend delivery of stimulation without intervention from INS 26,e.g., according to schedule stored by memory 92, where the schedule mayindicate the times at which INS 26 takes impedance measurements.

Processor 110 may determine a first electrical parameter valueindicative of an impedance of an electrical path (402), e.g., bygenerating and delivering a constant voltage signal or a constantcurrent signal and measuring a resulting current or voltage,respectively, of a sensed signal, respectively. The electrical path maycomprise, for example, a path between stimulation generator 114 andelectrodes 124 (FIG. 7) of lead 28. Thereafter, processor 90 mayactivate the delivery of stimulation signals by ICD 16 (404). Forexample, processor 90 of ICD 16 may control stimulation generator 94 togenerate and deliver stimulation upon the detection of an arrhythmia orat regular intervals, e.g., to pace heart 14.

While ICD 16 is delivering stimulation signals to patient 12, processor110 of INS 26 may determine a second electrical parameter valueindicative of the impedance of the electrical path (406). Processor 110may compare the first and second determined impedance values (408). Ifthe first and second determined impedance values are within a thresholdrange, e.g., within about 20% or less of each other, such as about 10%or substantially equal, processor 110 may determine that the delivery ofstimulation by ICD 16 does not adversely affect the impedancedetermination by INS 26. Processor 90 may periodically perform thetechnique shown in FIG. 27, such as at an impedance sampling frequencyof about 1 Hz to about 100 Hz or about one to about ten times perminute.

On the other hand, if the first and second determined impedance valuesare not within the threshold range of each other (408), processor 110may generate an impedance measurement interference indication (410). Theimpedance measurement interference indication may be a value, flag, orsignal that is stored in memory 112 of INS 26 or transmitted to anotherdevice (e.g., programmer 24 or ICD 16) to indicate that the delivery ofstimulation by ICD 16 may potentially be interfering with the accurateand precise impedance measurements of one or more electrical pathscoupled to INS 26.

In other examples, any part of the techniques shown in FIGS. 27-29 maybe performed by processor 130 of programmer 24 or another device.

In some cases, processor 90 of ICD 16, processor 110 of INS 26 oranother device may evaluate a change in the difference between the firstand second electrical parameter values over time to evaluate theintegrity of therapy system 10 (FIG. 1). As indicated above, the firstelectrical parameter value may be indicative of an impedance of anelectrical path electrically connected to ICD 16 or INS 26 while INS 26or ICD 16, respectively, is not actively delivering stimulation topatient 12, and the second electrical parameter value may be indicativeof the impedance of the electrical path while INS 26 or ICD 16,respectively, is delivering stimulation to patient 12.

FIG. 30 is a flow diagram illustrating an example technique forevaluating the integrity of therapy system 10 based on the differencebetween the first and second electrical parameter values over time.Processor 90 of ICD 16 or processor 110 of INS 26 may determine thedifference between the first and second electrical parameter values overtime (412). For example, for each impedance determination, e.g., asdescribed above with respect to FIG. 27, processor 90 or processor 110may determine the difference between the first and second electricalparameter values and store the value indicative of the difference inmemory 92 (FIG. 6). In other examples, processor 90 or processor 110 maydetermine the difference between the first and second electricalparameter values less frequently than the frequency with which the firstand second electrical parameter values are determined. For example,processor 90 or processor 110 may determine the difference between thefirst and second electrical parameter values once for every two timesthe first and second electrical parameter values are determined. Otherfrequencies with which processor 90 or processor 110 the differencebetween the first and second electrical parameter values arecontemplated.

Processor 90 or processor 110 may determine whether the differencebetween the first and second electrical parameter values is increasingover time (414). That is, processor 90 or processor 110 may determine atrend in a difference between the impedance of the electrical pathelectrically connected to ICD 16 that is determined while INS 26 isdelivering stimulation begins to differ from the impedance that isdetermined while INS 26 is not actively delivering stimulation topatient 12. This trend may indicate, for example, whether the crosstalkbetween ICD 16 and INS 26 is increasing over time. In addition, thetrend may indicate whether another system integrity issue, such as alead-related condition, may be present.

If the difference between first and second electrical parameter valuesremains substantially constant over time (e.g., stays within aparticular range, such as less than about 25% of a mean or mediandifference value), processor 90 or processor 110 may determine that asystem integrity issue is not present. Processor 90 or processor 110 maythen continue monitoring the difference between the first and secondelectrical parameter values over time (412).

On the other hand, if the difference between first and second electricalparameter values increases over time, processor 90 or processor 110 maydetermine that a therapy system integrity issue is present. Accordingly,processor 90 or processor 110 may generate a system integrity indication(416). The system integrity indication may be a value, flag, or signalthat is stored or transmitted to indicate that clinician attention isdesirable. The clinician attention may be desirable to, for example,assess the integrity of leads 18, 20, 22, 28, 29 that may be implantedwithin patient 12. In some examples, processor 90 or 110 may transmitthe system integrity indication to programmer 24 or another externaldevice, including remote devices, e.g., using a system described withrespect to FIG. 32.

In some examples, processor 90 or processor 110 may generate the systemintegrity indication if the difference between the first and secondelectrical parameter values increases over time by a predetermined rate,which may be stored in memory 92 or 112 of ICD 16 or INS 26,respectively. In other examples, processor 90 or processor 110 maygenerate the system integrity indication if the difference between thefirst and second electrical parameter values at a particular point intime exceeds the mean or median difference by a threshold value. Themean or median difference may be determined based on the mean or medianvalue of the difference between the first and second electricalparameter values over a particular range of time preceding the currentdetermination of the difference between the first and second electricalparameter values.

The techniques described herein, such as the techniques described withrespect to FIGS. 9-12B for modifying one or more operating parameters ofINS 26 in order to minimize crosstalk between INS 26 and ICD 16, withrespect to FIGS. 16, 19A, and 19B for modifying one or more sensingparameters of ICD 16 in order to minimize crosstalk between INS 26 andICD 16, with respect to FIGS. 20, 21, 23-30 for determining the extentof crosstalk between INS 26 and ICD 16, may also be implemented fordetermining the extent of crosstalk in a device comprising thefunctionality of INS 26 and ICD 16 in a common housing.

FIG. 31 is a functional block diagram illustrating an example IMD 420that includes a neurostimulation module 422 and a cardiac therapy module424 in a common housing 426. Neurostimulation therapy module 422includes stimulation generator 114, which is described above withrespect to FIG. 7. Similarly, cardiac therapy module 424 includesstimulation generator 94 and sensing module 96, which are describedabove with respect to FIG. 6. IMD 420 also includes processor 90, memory92, telemetry module 98, and power source 100, which are described abovewith respect to FIG. 6.

Neurostimulation therapy module 422 may deliver electrical stimulationto a tissue site proximate to a nerve. As previously discussed withrespect to INS 26, the stimulation may be delivered to the nerve via anintravascular lead or an extravascular lead. In other examples,neurostimulation therapy module 422 may deliver electrical stimulationto a nonmyocardial tissue site that may or may not be proximate a nerve.Cardiac therapy module 424 may sense electrical cardiac signals ofpatient 12 and deliver cardiac rhythm management therapy to heart 14,such as pacing, cardioversion or defibrillation therapy.

Processor 90 may control neurostimulation therapy module 422 and cardiactherapy module 424 according to any of the techniques described above tominimize the possibility that cardiac therapy module 424 deliverselectrical stimulation to heart 14 in response to detecting electricalsignals generated and delivered by neurostimulation therapy module 422that resemble an arrhythmic cardiac signal. For example, with respect tothe technique shown in FIG. 9, processor 90 may control neurostimulationtherapy module 422 to deliver stimulation therapy to patient 12 (140).In addition, processor 90 may control sensing module 96 to senseelectrical cardiac signals (142).

If processor 90 detects a potential arrhythmia based on the sensedelectrical cardiac signals (144), processor may modify the stimulationsignals delivered by neurostimulation therapy module 422 (146). Forexample, processor 90 may modify one or more therapy parameter valueswith which neurostimulation therapy module 422 generates electricalstimulation signals, e.g., using the techniques described with respectto FIGS. 11A-11D. As another example, processor 90 may switch thetherapy programs with which neurostimulation therapy module 422generates the electrical stimulation signals, e.g., using the techniquesdescribed with respect to FIGS. 12A and 12B.

Processor 90 of the IMD 420 including both neurostimulation therapymodule 422 and cardiac therapy module 424 may also modify one or moresensing parameters of sensing module 96 if neurostimulation therapymodule 422 is delivering electrical stimulation therapy to patient 12,e.g., as described with respect to FIGS. 16, 19A, and 19B.

Programmer 24 or another device may also evaluate the crosstalk betweenneurostimulation therapy module 422 and cardiac therapy module 424 usingany of the techniques described herein, e.g., the techniques describedwith reference to FIGS. 20, 21, and 23-26. However, instead ofcontrolling ICD 16 and INS 26 or receiving information from separatedevices 16, 26, programmer 24 may control neurostimulation therapymodule 422 and cardiac therapy module 424 of a common IMD 420, andreceive information from a single IMD 420. In addition, the techniquesshown in FIGS. 27-30 may also be implemented by processor 90 in order todetermine whether the delivery of electrical stimulation byneurostimulation therapy module 422 or cardiac therapy module 424interferes with impedance measurements taken by processor 90.

FIG. 32 is a block diagram illustrating a system 430 that includes anexternal device 432, such as a server, and one or more computing devices434A-434N that are coupled to ICD 16, INS 26, and programmer 24 shown inFIG. 1 via a network 436, according to one example. In this example, ICD16 and INS 26 uses their respective telemetry modules 98 (FIG. 6) and118 (FIG. 7) to communicate with programmer 24 via a first wirelessconnection, and to communicate with an access point 438 via a secondwireless connection. In the example of FIG. 11, access point 438,programmer 24, external device 432, and computing devices 434A-434N areinterconnected, and able to communicate with each other, through network436.

In some cases, one or more of access point 438, programmer 24, externaldevice 432, and computing devices 434A-434N may be coupled to network436 through one or more wireless connections. ICD 16, INS 26, programmer24, external device 432, and computing devices 434A-434N may eachcomprise one or more processors, such as one or more microprocessors,DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, that mayperform various functions and operations, such as those describedherein.

Access point 438 may comprise a device that connects to network 436 viaany of a variety of connections, such as telephone dial-up, digitalsubscriber line (DSL), or cable modem connections. In other examples,access point 438 may be coupled to network 436 through different formsof connections, including wired or wireless connections. In someexamples, access point 438 may communicate with programmer 24, ICD 16,and/or INS 26. Access point 438 may be co-located with patient 12 (e.g.,within the same room or within the same site as patient 12) or may beremotely located from patient 12. For example, access point 438 may be ahome monitor that is located in the patient's home or is portable forcarrying with patient 12.

During operation, ICD 16 and/or INS 26 may collect, measure, and storevarious forms of diagnostic data. For example, as described previously,ICD 16 or INS 26 may collect electrical parameter values indicative ofan impedance of an electrical path. In certain cases, ICD 16 or INS 26may directly analyze collected diagnostic data and generate anycorresponding reports or alerts. In some cases, however, ICD 16 or INS26 may send diagnostic data to programmer 24, access point 438, and/orexternal device 432, either wirelessly or via access point 438 andnetwork 436, for remote processing and analysis.

For example, ICD 16 or INS 26 may send programmer 24 collectedelectrical parameter values indicative of the impedance of variouselectrical paths of therapy system 10 (FIG. 1), arrhythmia indicationsthat indicate an arrhythmia was detected (e.g., as discussed withrespect to FIG. 10), interference indications that indicate modificationto the stimulation or sensing parameters of ICD 16 or INS 26 failed toreduce detected crosstalk between ICD 16 and INS 26 (e.g., as discussedwith respect to FIGS. 11A-11D), interference indications that indicatethe determined interference between ICD 16 and INS 26 or otherwisedetected exceeds a certain level (e.g., as discussed with respect toFIGS. 21 and 23), and impedance measurement interference indicationsthat indicate that stimulation delivery by ICD 16 or INS 26 may beinterfering with the measurement of the impedance of various electricalpaths of therapy system 10 (e.g., as discussed with respect to FIGS.27-29).

Processor 24 may analyze the received electrical parameter values and/orindications. Programmer 24 may generate reports or alerts afteranalyzing the information from ICD 16 or INS 26 and determine whetherthe values and indications indicate that patient 12 requires medicalattention, e.g., based on ICD 16 and INS 26 crosstalk that exceeds anacceptable level. In some cases, ICD 16, INS 26, and/or programmer 24may combine all of the diagnostic data into a single displayable report,which may be displayed on programmer 24. The report may containinformation concerning the impedance measurements or indications, thetime of day at which the measurements were taken or at which theindications were generated, and identify any patterns in the impedancemeasurements or arrhythmia or interference indications.

In another example, ICD 16 or INS 26 may provide external device 432with collected impedance data via access point 438 and network 436.External device 432 includes one or more processors 440. In some cases,external device 432 may request collected impedance data and storedindications, and in some cases, ICD 16 or INS 26 may automatically orperiodically provide such data to external device 432. Upon receipt ofthe impedance data and indication data via input/output device 442,external device 432 is capable of analyzing the data and generatingreports or alerts upon determination that the impedance data indicates alead integrity issue or upon determination that additional clinicianassistance is necessary to decrease the crosstalk between ICD 16 and INS26. In some examples, ICD 16 or INS 26 may analyze the data and generatereports or alerts, which may be transmitted to external device 432 vianetwork 436. In addition, in some examples, a therapy system may notinclude programmer 24 to evaluate crosstalk, but, may instead rely onexternal device 432 or other devices to evaluate crosstalk between ICD16 and INS 26.

In one example, external device 432 may combine the diagnostic data intoan report. One or more of computing devices 434A-434N may access thereport through network 436 and display the report to users of computingdevices 434A-434N. In some cases, external device 432 may automaticallysend the report via input/output device 442 to one or more of computingdevices 434A-434N as an alert, such as an audio or visual alert. In somecases, external device 432 may send the report to another device, suchas programmer 24, either automatically or upon request. In some cases,external device 432 may display the report to a user via input/outputdevice 442.

In one example, external device 432 may comprise a secure storage sitefor diagnostic information that has been collected from ICD 16, INS 26,and/or programmer 24. In this example, network 436 may comprise anInternet network, and trained professionals, such as clinicians, may usecomputing devices 434A-434N to securely access stored diagnostic data onexternal device 432. For example, the trained professionals may need toenter usernames and passwords to access the stored information onexternal device 432. In one example, external device 432 may be aCareLink server provided by Medtronic, Inc., of Minneapolis, Minn.

The examples therapy systems described herein include one ICD 16 and oneINS 26. In other examples, the techniques described herein may alsoapply to therapy systems that include more than one ICD 16 and/or morethan one INS 26. For example, the techniques shown in FIGS. 9-11D formodifying one or more electrical stimulation parameter values of an INSmay be applicable to modifying one or more electrical stimulationparameter values for more than one INS. Some therapy systems may includemore than one INS. For example, some therapy systems may includemultiple microstimulators that each delivers electrical stimulationtherapy to patient 12. A microstimulator may include a substantiallyself-contained electrical stimulation device that includes electrodes ona housing of the microstimulator, rather than being coupled toelectrodes via one or more leads that extend from the housing. However,the microstimulator may be coupled to electrodes of leads in someexamples. The multiple implanted microstimulators or other INS' may bedistributed throughout the patient's body. In some examples, themicrostimulators may communicate with each other to coordinate therapydelivery to patient 12. In addition, in some examples, themicrostimulators may communicate with a master microstimulator or ICD16, either of which may control the delivery of electrical stimulationby one or more of the other implanted microstimulators. Delivery ofelectrical stimulation signals by any one of the INS' may generatecrosstalk with ICD 16. Thus, the techniques described herein may be usedto minimize the crosstalk between one or more of the implanted INS' andICD 16, evaluate the crosstalk between one or more of the implanted INS'and ICD 16, and the like.

The techniques described in this disclosure, including those attributedto ICD 16, INS 26, programmer 24, or various constituent components, maybe implemented, at least in part, in hardware, software, firmware or anycombination thereof. For example, various aspects of the techniques maybe implemented within one or more processors, including one or moremicroprocessors, DSPs, ASICs, FPGAs, or any other equivalent integratedor discrete logic circuitry, as well as any combinations of suchcomponents, embodied in programmers, such as physician or patientprogrammers, stimulators, image processing devices or other devices. Theterm “processor” or “processing circuitry” may generally refer to any ofthe foregoing logic circuitry, alone or in combination with other logiccircuitry, or any other equivalent circuitry.

Such hardware, software, firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. While the techniques describedherein are primarily described as being performed by processor 90 of ICD16, processor 110 of INS 26, and/or processor 130 of programmer 24, anyone or more parts of the techniques described herein may be implementedby a processor of one of the devices 16, 26, programmer 24 or anothercomputing device, alone or in combination with ICD 16, INS 26 orprogrammer 24.

In addition, any of the described units, modules or components may beimplemented together or separately as discrete but interoperable logicdevices. Depiction of different features as modules or units is intendedto highlight different functional aspects and does not necessarily implythat such modules or units must be realized by separate hardware orsoftware components. Rather, functionality associated with one or moremodules or units may be performed by separate hardware or softwarecomponents, or integrated within common or separate hardware or softwarecomponents.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as RAM, ROM, NVRAM,EEPROM, FLASH memory, magnetic data storage media, optical data storagemedia, or the like. The instructions may be executed to support one ormore aspects of the functionality described in this disclosure.

Various examples have been described in the disclosure. These and otherexamples are within the scope of the following example statements.

1. A method comprising: determining a first electrical parameter valueindicative of an impedance of an electrical path coupled to a firstimplantable medical device (IMD); activating delivery of electricalstimulation to a patient by a second IMD after determining the firstelectrical parameter value; determining a second electrical parametervalue indicative of the impedance of the electrical path while thesecond IMD delivers electrical stimulation; determining whether thefirst and second electrical parameter values are within a thresholdrange of each other; and generating an indication if the first andsecond electrical parameter values are not within the threshold range ofeach other.
 2. The method of claim 1, further comprising, if the firstand second electrical parameter values are not within the thresholdrange of each other, modifying at least one stimulation parameter withwhich the second implantable medical device generates the electricalstimulation.
 3. The method of claim 2, wherein the at least onestimulation parameter comprises at least one of a current amplitude, avoltage amplitude, a pulse width, a pulse rate, a frequency, a dutycycle or an electrode combination.
 4. The method of claim 1, wherein thefirst implantable medical device (IMD) delivers at least one of pacing,cardioversion or defibrillation therapy to a heart of the patient, andthe second IMD delivers the electrical stimulation to a tissue sitewithin the patient.
 5. The method of claim 4, wherein the tissue sitecomprises at least one of a nonmyocardial tissue site or a nonvascularcardiac tissue site.
 6. The method of claim 4, wherein the tissue sitecomprises at least one of an extravascular tissue site or the tissuesite proximate to a nerve of the patient.
 7. The method of claim 1,wherein the first implantable medical device (IMD) delivers theelectrical stimulation to a tissue site within the patient and thesecond IMD delivers at least one of pacing, cardioversion ordefibrillation signal to a heart of the patient.
 8. The method of claim1, further comprising suspending delivery of electrical stimulation bythe second implantable medical device (IMD), wherein determining thefirst electrical parameter value comprises determining the firstelectrical parameter value while the delivery of electrical stimulationby the second IMD is suspended.
 9. The method of claim 1, furthercomprising adjusting the delivery of electrical stimulation by thesecond implantable medical device if the first and second electricalparameter values are not within the threshold range of each other. 10.The method of claim 9, wherein adjusting the delivery of electricalstimulation by the second implantable medical device (IMD) comprisessuspending the delivery of electrical stimulation by the second IMD. 11.The method of claim 1, wherein the threshold range comprises about fivepercent to about 20 percent.
 12. The method of claim 1, whereingenerating the indication comprises generating at least one of anauditory, visual or somatosensory notification.
 13. The method of claim1, wherein the indication comprises a first indication, the methodfurther comprising: determining a difference between the first andelectrical parameter values; determining a trend in the differencebetween the first and electrical parameter values over time; andgenerating a second indication based on the trend.
 14. The method ofclaim 13, wherein generating the second indication based on the trendcomprises generating the second indication if the difference between thefirst and electrical parameter values increases over time.
 15. A systemcomprising: a first implantable medical device (IMD) implanted within apatient; a second IMD implanted within the patient; and a processor thatdetermines a first electrical parameter value indicative of an impedanceof an electrical path coupled to the first IMD, determines a secondelectrical parameter value indicative of the impedance of the electricalpath while the second IMD delivers electrical stimulation to thepatient, determines whether the first and second electrical parametervalues are within a threshold range of each other, and generates anindication if the first and second electrical parameter values are notwithin the threshold range of each other.
 16. The system of claim 15,wherein, if the first and second electrical parameter values are notwithin the threshold range of each other, the processor initiates amodification to at least one stimulation parameter with which the secondimplantable medical device generates the electrical stimulation.
 17. Thesystem of claim 16, wherein the processor initiates the modification tothe at least one stimulation parameter by at least providing a controlsignal to the second implantable medical device (IMD), wherein thesecond IMD modifies the at least one stimulation parameter.
 18. Thesystem of claim 16, wherein the processor initiates the modification tothe at least one stimulation parameter by at least providing themodified at least one stimulation parameter to the second implantablemedical device.
 19. The system of claim 15, wherein the firstimplantable medical device (IMD) delivers at least one of pacing,cardioversion or defibrillation therapy to a heart of the patient, andthe second IMD delivers the electrical stimulation to a tissue sitewithin the patient.
 20. The system of claim 19, wherein the tissue sitecomprises at least one of a nonmyocardial tissue site or a nonvascularcardiac tissue site.
 21. The system of claim 19, wherein the tissue sitecomprises at least one of an extravascular tissue site or the tissuesite proximate to a nerve of the patient.
 22. The system of claim 15,wherein the first implantable medical device (IMD) delivers theelectrical stimulation to a tissue site within the patient and thesecond IMD delivers at least one of pacing, cardioversion ordefibrillation therapy to a heart of the patient.
 23. The system ofclaim 15, wherein the processor controls the second implantable medicaldevice to suspend the delivery of electrical stimulation if the firstand second electrical parameter values are not within a threshold rangeof each other.
 24. The system of claim 15, wherein the processorcontrols the second implantable medical device (IMD) to suspend thedelivery of electrical stimulation and the processor determines thefirst electrical parameter value while the delivery of electricalstimulation by the second IMD is suspended.
 25. The system of claim 15,wherein the threshold range comprises about five percent to about 20percent.
 26. The system of claim 15, wherein the indication comprises afirst indication, and the processor determines a difference between thefirst and electrical parameter values, determines a trend in thedifference between the first and electrical parameter values over time,and generates a second indication based on the trend.
 27. The system ofclaim 26, wherein the processor generates the second indication based onthe trend by at least generating the second indication if the differencebetween the first and electrical parameter values increases over time.28. The system of claim 15, further comprising a medical deviceprogrammer, wherein the processor presents the indication via a userinterface of the medical device programmer.
 29. A system comprising:means for determining a first electrical parameter value indicative ofan impedance of an electrical path coupled to a first implantablemedical device (IMD); means for activating delivery of electricalstimulation to a patient by a second IMD after determining the firstelectrical parameter value; means for determining a second electricalparameter value indicative of the impedance of the electrical path whilethe second IMD delivers electrical stimulation; means for determiningwhether the first and second electrical parameter values are within athreshold range of each other; and means for generating an indication ifthe first and second electrical parameter values are not within thethreshold range of each other.
 30. The system of claim 29, furthercomprising means for modifying at least one stimulation parameter withwhich the second implantable medical device generates the electricalstimulation if the first and second electrical parameter values are notwithin the threshold range of each other.
 31. A computer-readable mediumcomprising instructions that cause a programmable processor to:determine a first electrical parameter value indicative of an impedanceof an electrical path coupled to a first implantable medical device(IMD); activate delivery of electrical stimulation to a patient by asecond IMD after determining the first electrical parameter value;determine a second electrical parameter value indicative of theimpedance of the electrical path while the second IMD deliverselectrical stimulation; determine whether the first and secondelectrical parameter values are within a threshold range of each other;and generate an indication if the first and second electrical parametervalues are not within the threshold range of each other.